U.S. patent number 9,412,852 [Application Number 14/606,014] was granted by the patent office on 2016-08-09 for low-temperature fabrication of nanomaterial-derived metal composite thin films.
This patent grant is currently assigned to Northwestern University, Polyera Corporatoin. The grantee listed for this patent is Northwestern University, Polyera Corporation. Invention is credited to Antonio Facchetti, Mercouri G. Kanatzidis, Myung-Gil Kim, Tobin J. Marks, William Christopher Sheets, Yu Xia, He Yan.
United States Patent |
9,412,852 |
Facchetti , et al. |
August 9, 2016 |
Low-temperature fabrication of nanomaterial-derived metal composite
thin films
Abstract
Disclosed are new methods of fabricating nanomaterial-derived
metal composite thin films via solution processes at low
temperatures (<400.degree. C.). The present thin films are
useful as thin film semiconductors, thin film dielectrics, or thin
film conductors, and can be implemented into semiconductor devices
such as thin film transistors and thin film photovoltaic
devices.
Inventors: |
Facchetti; Antonio (Chicago,
IL), Marks; Tobin J. (Evanston, IL), Kanatzidis; Mercouri
G. (Wilmette, IL), Kim; Myung-Gil (Evanston, IL),
Sheets; William Christopher (Chicago, IL), Yan; He (Hong
Kong, CN), Xia; Yu (Skokie, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
Northwestern University
Polyera Corporation |
Evanston
Skokie |
IL
IL |
US
US |
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Assignee: |
Northwestern University
(Evanston, IL)
Polyera Corporatoin (Skokie, IL)
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Family
ID: |
45888462 |
Appl.
No.: |
14/606,014 |
Filed: |
January 26, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150206957 A1 |
Jul 23, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13804037 |
Mar 14, 2013 |
8940578 |
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PCT/US2012/023042 |
Jan 27, 2012 |
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61462125 |
Jan 28, 2011 |
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61502644 |
Jun 29, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
21/02244 (20130101); H01L 21/02178 (20130101); H01L
29/78693 (20130101); H01L 21/02565 (20130101); C23C
18/127 (20130101); H01L 29/7869 (20130101); C23C
18/1237 (20130101); H01L 21/02205 (20130101); C23C
18/1216 (20130101); H01L 29/66969 (20130101); H01L
29/45 (20130101); H01L 21/28506 (20130101); C23C
18/1225 (20130101); H01L 21/441 (20130101); H01L
21/477 (20130101); H01L 29/66742 (20130101); H01L
21/02192 (20130101); H01L 21/02614 (20130101); C23C
18/125 (20130101); H01L 29/517 (20130101); H01L
21/02172 (20130101); H01L 21/443 (20130101) |
Current International
Class: |
H01L
21/441 (20060101); H01L 21/285 (20060101); H01L
29/786 (20060101); H01L 21/443 (20060101); H01L
21/477 (20060101); H01L 29/45 (20060101); C23C
18/12 (20060101); H01L 21/02 (20060101); H01L
29/66 (20060101); H01L 29/51 (20060101) |
Field of
Search: |
;438/608,778 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Basu et al: "Liquid-Phase Deposition of A1.sub.2 O.sub.3 Thin Films
on GaN," Journal of the Electrochemical Society, vol. 154, No. 12,
pp. H1041-H1046 (2007). cited by applicant .
Biswas et al: "Surface Characterization of Sol-Gel Derived Indium
Tin Oxide Films on Glass," Bull. Mater. Sci., vol. 29, No. 3, pp.
323-330 (2006). cited by applicant .
Biswas et al: "Work Function of Sol-Gel Indium Tin Oxide (ITO)
Films on Glass," Applied Surface Science, vol. 253, pp. 1953-1959
(2006). cited by applicant .
Chu et al: "Plasma-Enhanced Flexible Metal-Insulator-Metal
Capacitor Using High-k ZrO.sub.2 Film as Gate Dielectric with
Improved Reliability," Microelectronics Reliability, vol. 50, pp.
1098-1102 (2010). cited by applicant .
Exarhos et al: "Deposition and Characterization of Multicomponent
Oxide Films and Multilayers from Aqueous Solution" Thin Solid
Films, vol. 236, pp. 51-57 (1993). cited by applicant .
Kim et al.: "Low-Temperature Fabrication of High-Performance Metal
Oxide Thin-Film Electronics Via Combustion Procession" Nature
Materials, vol. 10, pp. 382-388 (Apr. 17, 2011). cited by applicant
.
Lee et al.: "Electrical and Optical Properties of In.sub.2 O.sub.3
-ZnO Thin Films Prepared by Sol-Gel Method," Thin Solid Films, vol.
484, pp. 184-187 (2005). cited by applicant.
|
Primary Examiner: Enad; Christine
Attorney, Agent or Firm: Chan; Karen K.
Government Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with government support under
FA9550-08-1-0331 awarded by the Air Force Office of Scientific
Research and DMR0520513 awarded by the National Science Foundation.
The government has certain rights in the invention.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser.
No. 13/804,037, filed on Mar. 14, 2013, which is a continuation of
International Application No. PCT/US2012/023042, filed on Jan. 27,
2012, which claims priority to and the benefit of U.S. Provisional
Patent Application Ser. No. 61/462,125, filed on Jan. 28, 2011, and
U.S. Provisional Patent Application Ser. No. 61/502,644, filed on
Jun. 29, 2011, the disclosure of each of which is incorporated by
reference herein in its entirety.
Claims
What is claimed is:
1. A method of fabricating a transistor device comprising a thin
film semiconductor component comprising indium gallium zinc oxide
(IGZO), the method comprising: depositing a thin film from a
nanomaterial dispersion comprising a semiconducting metal oxide
nanomaterial in a solvent or solvent mixture and a binder
composition comprising an indium salt, a gallium salt, and a zinc
salt in a solvent or solvent mixture, wherein the indium salt, the
gallium salt, and the zinc salt independently comprise either an
oxidizing anion or a fuel anion, provided that (a) if none of the
indium salt, the gallium salt and the zinc salt comprises a fuel
anion, then the binder composition further comprises either a fuel
or an ammonium salt comprising a fuel anion, and (b) if none of the
indium salt, the gallium salt and the zinc salt comprises an
oxidizing anion, then the binder composition further comprises
either an acid comprising an oxidizing anion or an inorganic salt
comprising an oxidizing anion, wherein: the fuel is selected from
acetylacetone, CF.sub.3COCH.sub.2COCF.sub.3,
CH.sub.3COCHFCOCH.sub.3, CH.sub.3COCH.sub.2C(.dbd.NH)CF.sub.3,
CH.sub.3C(.dbd.NH)CHFC(.dbd.NH)CH.sub.3,
CH.sub.3COCH.sub.2C(.dbd.NCH.sub.3)CF.sub.3,
CH.sub.3C(.dbd.NCH.sub.3)CHFC(.dbd.NCH.sub.3)CH.sub.3,
CH.sub.3C(.dbd.NH)CHFC(.dbd.NCH.sub.3)CH.sub.3,
Ph.sub.2POCH.sub.2COCH.sub.3, urea, N-methylurea, citric acid,
ascorbic acid, stearic acid, nitromethane, hydrazine,
carbohydrazide, oxalyl dihydrazide, malonic acid dihydrazide, tetra
formal tris azine, hexamethylenetetramine, and malonic anhydride,
the fuel anion is selected from acetylacetonate, a citrate, an
oxalate, an ascorbate, and a stearate, and the oxidizing anion is
selected from a nitrate, a perchlorate, a chlorate, a hypochlorite,
an azide, a peroxide, a superoxide, a high-valent oxide, an
N-oxide, a persulfate, a dinitramide, a nitrocyanamide, a
nitroarylcarboxylate, a tetrazolate, and hydrates thereof, and the
oxidizing anions and the fuel and fuel anions in the binder
composition are present in amounts to allow metal oxide formation
and complete combustion of the fuel and fuel anions; and annealing
the thin film at a temperature of less than or about 350.degree. C.
to initiate a combustion reaction between the oxidizing anions and
the fuel and fuel anions, thereby forming a thin film semiconductor
component comprising IGZO and converting the fuel and fuel anions
into CO.sub.2, H.sub.2O, and optionally, N.sub.2; wherein the
transistor device exhibits a charge mobility of about 3 cm.sup.2/Vs
or higher.
2. The method of claim 1, wherein the nanomaterial dispersion and
the binder composition are deposited separately.
3. The method of claim 1, wherein the nanomaterial dispersion and
the binder composition are combined before the depositing step.
4. The method of claim 1, wherein the semiconducting metal oxide
nanomaterial is selected from a semiconducting metal oxide
nanoparticle, a semiconducting metal oxide nanosphere, a
semiconducting metal oxide nanowire, a semiconducting metal oxide
nanoribbon, a semiconducting metal oxide nanorod, a semiconducting
metal oxide nanotube, a semiconducting metal oxide nanosheet, and
mixtures thereof.
5. The method of claim 1, wherein the semiconducting metal oxide
nanomaterial comprises an IGZO nanomaterial.
6. The method of claim 1, wherein the nanomaterial dispersion
comprises a solid loading between about 5 mg/mL and about 500
mg/mL.
7. The method of claim 1, further comprising providing a metal
oxide thin film dielectric, wherein providing the metal oxide thin
film dielectric comprises: depositing a thin film from a
nanomaterial dispersion comprising an electrically insulating metal
oxide nanomaterial in a solvent or solvent mixture and a dielectric
binder composition comprising a fuel and one or more oxidizing
agents in a solvent or solvent mixture, wherein the fuel and/or at
least one of the oxidizing agent(s) comprise a metal salt
comprising aluminum or cerium, and wherein the fuel and the one or
more oxidizing agents are present in amounts to allow metal oxide
formation and complete combustion of the fuel and fuel anions; and
annealing the thin film at a temperature less than or about
350.degree. C. with or without exposure to a radiation source,
thereby providing a metal oxide thin film dielectric and converting
the fuel and fuel anions into CO.sub.2, H.sub.2O, and optionally,
N.sub.2.
8. The method of claim 7, wherein the electrically insulating metal
oxide nanomaterial comprises Al.sub.2O.sub.3 nanoparticles,
ZrO.sub.2 nanoparticles, or HfO.sub.2 nanoparticles.
9. The method of claim 1, further comprising providing a metal
oxide thin film conductor, wherein providing the metal oxide thin
film conductor comprises: depositing a thin film from a
nanomaterial dispersion comprising an electrically conducting metal
oxide nanomaterial in a solvent or solvent mixture and a conductor
binder composition comprising a fuel and one or more oxidizing
agents in a solvent or solvent mixture, wherein the fuel and/or at
least one of the oxidizing agent(s) comprise a metal salt, and
wherein the fuel and the one or more oxidizing agents are present
in amounts to allow metal oxide formation and complete combustion
of the fuel and fuel anions; and annealing the thin film at a
temperature less than or about 350.degree. C. with or without
exposure to a radiation source, thereby providing a metal oxide
thin film conductor and converting the fuel and fuel anions into
CO.sub.2, H.sub.2O, and optionally, N.sub.2.
10. The method of claim 9, wherein the electrically conducting
metal oxide nanomaterial comprises ITO nanoparticles or ITO
nanorods.
11. The method of claim 1, wherein the solvent or solvent mixture
in the binder composition comprises an alkoxyalcohol.
12. The method of claim 1, wherein the annealing step is performed
at a temperature of less than or about 250.degree. C.
13. The method of claim 1, wherein the annealing step is performed
at a temperature of less than or about 150.degree. C.
14. The method of claim 1, further comprising providing a metal
oxide thin film dielectric, wherein providing the metal oxide thin
film dielectric comprises: depositing a thin film from a dielectric
precursor composition comprising a fuel and one or more oxidizing
agents in a solvent or solvent mixture, wherein the fuel and/or at
least one of the oxidizing agent(s) comprise a metal salt
comprising zirconium or hafnium, and wherein the fuel and the one
or more oxidizing agents are present in substantially
stoichiometric amounts; and annealing the thin film at a
temperature less than or about 350.degree. C. with or without
exposure to a radiation source, to provide a metal oxide thin film
dielectric.
15. The method of claim 1, further comprising providing a metal
oxide thin film conductor, wherein providing the metal oxide thin
film conductor comprises: depositing a thin film from a conductor
precursor composition comprising a fuel and one or more oxidizing
agents in a solvent or solvent mixture, wherein the fuel and/or at
least one of the oxidizing agent(s) comprise a metal salt, and
wherein the fuel and the one or more oxidizing agents are present
in substantially stoichiometric amounts; and annealing the thin
film at a temperature less than or about 350.degree. C. with or
without exposure to a radiation source, to provide a metal oxide
thin film conductor.
16. The method of claim 1 further comprising coupling the thin film
semiconductor component comprising IGZO directly to an organic
layer.
17. The method of claim 1, wherein the depositing step is carried
out by spin-coating, slot-coating, drop-casting, zone casting, dip
coating, blade coating, spray-coating, rod coating, or
stamping.
18. The method of claim 1, wherein each of the indium salt, the
gallium salt, and the zinc salt comprises an oxidizing anion
selected from a nitrate, a perchlorate, a chlorate, a hypochlorite,
an azide, a peroxide, a superoxide, a high-valent oxide, an
N-oxide, a persulfate, a dinitramide, a nitrocyanamide, a
nitroarylcarboxylate, a tetrazolate, and hydrates thereof, and the
precursor composition further comprises a fuel selected from
acetylacetone, CF.sub.3COCH.sub.2COCF.sub.3,
CH.sub.3COCHFCOCH.sub.3, CH.sub.3COCH.sub.2C(.dbd.NH)CF.sub.3,
CH.sub.3C(.dbd.NH)CHFC(.dbd.NH)CH.sub.3,
CH.sub.3COCH.sub.2C(.dbd.NCH.sub.3)CF.sub.3,
CH.sub.3C(.dbd.NCH.sub.3)CHFC(.dbd.NCH.sub.3)CH.sub.3,
CH.sub.3C(.dbd.NH)CHFC(.dbd.NCH.sub.3)CH.sub.3,
Ph.sub.2POCH.sub.2COCH.sub.3, urea, N-methylurea, citric acid,
ascorbic acid, stearic acid, nitromethane, hydrazine,
carbohydrazide, oxalyl dihydrazide, malonic acid dihydrazide, tetra
formal tris azine, hexamethylenetetramine, and malonic
anhydride.
19. The method of claim 18, wherein each of the indium salt, the
gallium salt, and the zinc salt comprises nitrate or a hydrate
thereof.
20. The method of claim 19, wherein the binder composition further
comprises a fuel selected from acetylacetone,
CF.sub.3COCH.sub.2COCF.sub.3, CH.sub.3COCHFCOCH.sub.3,
CH.sub.3COCH.sub.2C(.dbd.NH)CF.sub.3,
CH.sub.3C(.dbd.NH)CHFC(.dbd.NH)CH.sub.3,
CH.sub.3COCH.sub.2C(.dbd.NCH.sub.3)CF.sub.3,
CH.sub.3C(.dbd.NCH.sub.3)CHFC(.dbd.NCH.sub.3)CH.sub.3,
CH.sub.3C(.dbd.NH)CHFC(.dbd.NCH.sub.3)CH.sub.3,
Ph.sub.2POCH.sub.2COCH.sub.3, urea, N-methylurea, citric acid,
ascorbic acid, stearic acid, nitromethane, hydrazine,
carbohydrazide, oxalyl dihydrazide, malonic acid dihydrazide, tetra
formal tris azine, hexamethylenetetramine, and malonic anhydride.
Description
BACKGROUND
Recent research efforts on macroelectronics promise revolutionary
applications such as transparent, flexible flat panel displays,
solar cells, and large area sensor arrays. The emerging materials
to achieve these functions include organic semiconductors,
carbon-based semiconductors, and various inorganic nanomaterials.
However, metal oxide electronics hold perhaps the greatest promise,
with demonstrated large-area compatibility and impressive device
performance (electron mobility up to .about.100 cm.sup.2/Vs)
compared to the current dominant hydrogenated amorphous silicon
(a-Si:H) technology (electron mobility .about.1 cm.sup.2/Vs). These
promising properties have triggered efforts to fabricate oxide
electronics on flexible plastic substrates compatible with
low-cost, high-throughput solution-processing.
Despite its promise, solution processing of metal oxide electronics
typically requires high annealing temperatures (e.g.,
T.sub.anneal.gtoreq.400.degree. C.). For example, in conventional
sol-gel approaches, a metal precursor (e.g., a metal salt) is used
in combination with a base (typically a Bronsted base as a
catalyst) and an organic stabilizer (e.g., a ligand compound) to
synthesize a sol solution, which is then spun on the substrate. The
metal precursor, typically metal alkoxides or metal chlorides,
undergoes hydrolysis and polycondensation reactions to form a gel.
Formation of the metal oxide involves connecting the metal centers
with oxo (M-O-M) bridges, therefore generating metal-oxo and, for
partial connections, metal-hydroxo (M-OH.cndot..cndot..cndot.HO-M)
polymers in solution. To densify the metal oxide, that is, to
convert the (M-OH.cndot..cndot..cndot.HO-M) to (M-O-M) lattice by
eliminating H.sub.2O and to completely burn off the organic portion
of the metal oxide film, sintering at high temperatures
(.gtoreq.400.degree. C.) is necessary. As such, current
solution-phase techniques employed to process metal oxide films
generally are incompatible with inexpensive flexible plastic
substrates. The limitations posed by these high processing
temperatures have prevented oxide materials from being implemented
in large-area flexible macroelectronics.
Furthermore, when metal oxide thin films are derived from
pre-formed nanomaterial solutions, an organic ligand similarly is
needed to stabilize the solution. Thus after spin-coating,
sintering of the resulting nanomaterial films at high temperatures
(e.g., >400.degree. C.) again is necessary to remove the ligand.
Otherwise, the residual ligands within the film can lead to poor
morphological and electrical connections between the nanomaterials,
hindering charge carrier transport and limiting the corresponding
device performance.
Accordingly, there is a need in the art for new precursor systems
and solution-phase processes that can be used to fabricate
electronic metal oxide thin films at low temperatures.
SUMMARY
In light of the foregoing, the present teachings provide methods
that can be used to achieve the solution deposition of diverse
electronic metal oxide films at low temperatures. In particular,
the present methods can be used to prepare electrically functional
metal oxide films at annealing temperatures generally below about
400.degree. C., and as low as about 150.degree. C. The present
teachings also relate to the implementation of the resulting films
in various semiconductor devices. In particular, high-performance
transistors on inexpensive and/or flexible substrates can be
achieved by implementing the present metal oxide films as the
electrically transporting (e.g., the semiconductor and/or any of
the source, drain, and gate electrode(s)) and/or the electrically
insulating (e.g., the gate dielectric) component(s). Other
semiconductor devices can include solar cells, sensors,
light-emitting transistors and circuits.
In one aspect, the present teachings provide formulations that can
be used to prepare metal oxide thin film semiconductors, metal
oxide thin film dielectrics, and/or metal oxide thin film
conductors via solution-phase processes at low temperatures.
A formulation (or precursor composition) according to the present
teachings generally includes a fuel and one or more oxidizing
agents in a solvent or solvent mixture, where the fuel and the
oxidizing agent(s) form a redox pair, and where the fuel and each
of the oxidizing agent(s) are at least partially soluble in the
solvent or solvent mixture. The oxidizing agent can be a metal
reagent comprising an oxidizing anion, an inorganic reagent
comprising an oxidizing anion, or an oxidizing acid. The fuel
generally can be described as an organic compound or organic anion
capable of releasing energy (i.e., heat) by the process of
oxidation. Accordingly, the fuel can be an organic fuel, a metal
reagent comprising a fuel anion, or an inorganic reagent comprising
a fuel anion. The solvent or solvent mixture can include an
alcohol. In some embodiments, either the oxidizing agent and/or the
fuel can comprise a metal reagent comprising a metal selected from
a Group 12 metal, a Group 13 metal, a Group 14 metal, a Group 15
metal, a transition metal (any Group 3 to Group 11 metal), or a
lanthanide. In other embodiments where neither the oxidizing agent
nor the fuel comprises a metal reagent, a metal salt (e.g., a metal
halide) can be added to the precursor composition in addition to
the fuel and the oxidizing agent. The precursor composition also
can include a base.
Unlike conventional precursors based on sol-gel approaches, which
convert the precursor into metal oxides via an endothermic
reaction, the oxidizing agent and the fuel in the present
formulations react to induce a self-energy generating combustion.
The self-generated heat from the precursors' reaction provides a
localized energy supply, thereby eliminating the need for high,
externally applied processing temperatures to drive the completion
of the metal oxide lattice. Therefore, the present formulations
allow metal oxide formation at temperatures much lower than through
the use of conventional sol-gel precursors.
More generally, the present teachings relate to a method of
preparing a metal oxide thin film that can be used as an
electrically transporting or insulating component in a
semiconductor device. The present method can include (a) depositing
a thin film from a precursor composition described herein; (b)
annealing the thin film at a temperature less than or about
350.degree. C.; and carrying out steps (a) and (b) one or more
times by depositing an overlying thin film on any previously
deposited and annealed thin film, where each film formed by a
single cycle of steps (a) and (b) has a thickness of less than or
about 50 nm, thereby achieving a fully dense metal oxide thin film.
The annealing step can be carried out by conventional heat
treatment (e.g., in an oven) and/or by exposure to a radiation
source (e.g., infrared (IR) radiation, ultraviolet radiation,
eximer laser radiation, microwave radiation).
The present low temperature-processed metal oxide thin films can be
incorporated into articles of manufacture such as field effect
transistors (e.g., thin film transistors), photovoltaics, organic
light emitting devices such as organic light emitting diodes
(OLEDs) and organic light emitting transistors (OLETs),
complementary metal oxide semiconductors (CMOSs), complementary
inverters, D flip-flops, rectifiers, ring oscillators, solar cells,
photovoltaic devices, photodetectors, and sensors. The present low
temperature-processed metal oxide thin films can provide
advantageous field-effect mobilities, which, without wishing to be
bound by any particular theory, can be achieved through improved
film texturing and/or interfacial and related morphological
considerations.
The present low temperature solution-processed metal oxide thin
films can be combined with metal oxide (e.g., IGZO), nitride (e.g.,
Si.sub.3N.sub.4), arsenite (e.g., GaAs), or organic semiconductor
(e.g., rylenes, donor-acceptor blends) films deposited via other
solution-phase processes or conventional methods such as thermal
evaporation and various physical and chemical vapor deposition
techniques (e.g., sputtering, plasma-enhanced chemical vapor
deposition (PECVD), atomic layer deposition (ALD), pulsed laser
deposition (PLD), and ion-assisted deposition (IAD)) to produce
hybrid multilayers.
Accordingly, the present teachings also can relate to a method of
fabricating a thin film transistor that includes a thin film
semiconductor, a thin film dielectric, a thin film gate electrode,
and source and drain electrodes, where the method can include
coupling the thin film semiconductor to the thin film dielectric
and coupling the thin film dielectric to the thin film gate
electrode. Specifically, the thin film semiconductor can be coupled
to the thin film dielectric by contacting the thin film dielectric
with a semiconductor precursor composition, where the semiconductor
precursor composition can include at least one oxidizing agent and
a fuel in a solvent or solvent mixture as described herein. The
semiconductor precursor composition in contact with the thin film
dielectric can be annealed to a temperature of less than or about
350.degree. C. to provide a metal oxide thin film semiconductor
having a thickness of less than or about 50 nm. Following the
annealing step, a new cycle including the contacting step and the
annealing step can be repeated one or more times to increase the
thickness of the final metal oxide thin film semiconductor.
In another aspect, the present teachings provide
nanomaterial-derived metal oxide or metal oxide/metal composite
thin films having electronic properties suitable for use in various
semiconductor devices. More specifically, the present teachings
provide metal oxide thin films deposited from either a single
dispersion or separate dispersions of a metal oxide nanomaterial
and a binder composition that includes the combustion precursors
(i.e., an oxidizing agent and a fuel) as described herein. The
metal oxide nanomaterial can be selected from a metal oxide
nanoparticle, a metal oxide nanosphere, a metal oxide nanowire, a
metal oxide nanoribbon, a metal oxide nanorod, a metal oxide
nanotube, a metal oxide nanosheet, and combinations thereof. The
metal component of the metal oxide nanomaterial can include a Group
12 metal, a Group 13 metal, a Group 14 metal, a Group 15 metal, a
transition metal, or a lanthanide. In various embodiments, either
the oxidizing agent and/or the fuel can comprise the same metal as
the metal oxide nanomaterial. The dispersion(s) of the metal oxide
nanomaterial and/or the oxidizing agent(s) and the fuel can include
a solvent or a solvent mixture that includes an alcohol.
In some embodiments, the nanomaterial-derived metal oxide or metal
oxide/metal composite thin film can be prepared from separate
dispersions of the nanomaterial and the binder composition. More
specifically, a first thin film layer can be deposited from a first
dispersion including the metal oxide or metal nanomaterial in a
solvent or a solvent mixture. This first thin film layer can have a
typical thickness ranging from about 5 nm to about 1000 nm, which
optionally can be annealed at a temperature of less than or about
350.degree. C. under an oxygen-containing, reducing, or inert
atmosphere. Following the deposition of the nanomaterial
dispersion, a binder composition which includes the combustion
precursors can be deposited one or more times (i.e., in one or more
deposition cycles) to provide a second thin film layer that has a
typical thickness ranging from about 5 nm to about 100 nm. This
binder composition can help fill the physical gaps between
nanomaterials. In various embodiments, each deposition cycle can
provide a binder sublayer that has a typical thickness of less than
or about 50 nm. In some embodiments, an annealing step can be
performed between each two deposition cycles. In other embodiments,
the second thin film layer is annealed once at the end of the
multiple deposition cycles. The annealing step(s) typically are
performed at a temperature of less than or about 350.degree. C.
The foregoing as well as other features and advantages of the
present teachings will be more fully understood from the following
figures, description, examples, and claims.
BRIEF DESCRIPTION OF DRAWINGS
It should be understood that the drawings described below are for
illustration purposes only. The drawings are not necessarily to
scale, with emphasis generally being placed upon illustrating the
principles of the present teachings. The drawings are not intended
to limit the scope of the present teachings in any way.
FIG. 1 compares differential thermal analysis (DTA, top row) and
thermogravimetric analysis (TGA, bottom row) data of combustion
precursors according to the present teachings (solid) versus
conventional precursors (dotted) of the following metal oxides: (a)
In.sub.2O.sub.3, (b) ZTO, (c) IZO, and (d) ITO.
FIG. 2 shows grazing incident angle x-ray diffraction (GIAXRD) data
of various oxide films deposited using combustion precursors
according to the present teachings or conventional precursors and
annealed at the indicated temperatures: (a) In.sub.2O.sub.3 film
deposited using combustion precursors; (b) In.sub.2O.sub.3 film
deposited using conventional precursors; (c) ITO deposited using
combustion precursors; (d) IZO film deposited using combustion
precursors; and (e) ZTO film deposited using combustion
precursors.
FIG. 3 shows x-ray photoelectron spectroscopy (XPS) spectra of
In.sub.2O.sub.3 films deposited with (a) combustion precursors
according to the present teachings and (b) conventional precursors
and annealed at the indicated temperatures (530.1 eV: M-O-M lattice
oxygen, 531.1 eV: M-OH metal hydroxide oxygen, 532.3 eV: adsorbed
oxygen species).
FIG. 4 shows XPS spectra of In.sub.2O.sub.3 films deposited with
(a) combustion precursors according to the present teachings and
(b) conventional precursors and annealed at 400.degree. C.,
250.degree. C., 200.degree. C., and 150.degree. C. (top to
bottom).
FIG. 5 shows scanning electron microscopy (SEM) images of (a) an 80
nm-thick combustion precursor-derived In.sub.2O.sub.3 film obtained
by deposition of a single layer (NH.sub.3/In=4) and (b) an 80
nm-thick combustion precursor-derived In.sub.2O.sub.3 film obtained
by deposition of four 20 nm-thick layers (NH.sub.3/In=2).
FIG. 6 compares the mobilities (.mu..sup.sat) of (a)
In.sub.2O.sub.3, (b) ZTO, and (c) IZO thin film transistors derived
from combustion precursors (circle symbols) versus conventional
precursors (square symbols) fabricated at different annealing
temperatures.
FIG. 7 compares the conductivities of ITO films derived from
combustion precursors (circle symbols) versus conventional
precursors (square symbols) fabricated at different annealing
temperatures.
FIG. 8 provides characterization data of amorphous alumina films
annealed at 200.degree. C. and 250.degree. C.: (a) C-V measurements
at 10 kHz; (b) leakage current measurements; (c) frequency
dependence measurements at 3V; and (d) GIAXRD spectra.
FIG. 9 shows representative transfer and output plots of combustion
precursor-derived TFTs with 2000 .mu.m channel width, 100 .mu.m
channel length, Al electrodes: (a) transfer plots of combustion
precursor-derived In.sub.2O.sub.3, ZTO, and IZO TFT devices
annealed at 250.degree. C. on 300 nm SiO.sub.2/p+Si substrates; (b)
transfer plots of a representative combustion precursor-derived
In.sub.2O.sub.3 TFT device on n++Si/.alpha.-alumina dielectric (38
nm .alpha.-alumina annealed at 250.degree. C., 41 nm
.alpha.-alumina annealed at 200.degree. C.); and (c) representative
transfer plot and (d) representative output plot of an all-oxide
TFT (In.sub.2O.sub.3/38 nm .alpha.-alumina/250 nm ITO/1737 F glass)
annealed at 250.degree. C.
FIG. 10 shows the (a) transfer plot and (b) output plot of a
representative flexible combustion precursor-derived metal oxide
TFT device (In.sub.2O.sub.3/41 nm .alpha.-alumina dielectric/30 nm
Al gate electrode/30 nm AryLite.TM. substrate).
FIG. 11 shows representative transfer plots of combustion
precursor-derived IGZO TFT devices annealed at different relative
humidities.
FIG. 12 shows representative transfer and output plots for
ZnO-based TFTs having the structure Si/300 nm SiO.sub.2/ZnO/100 nm
Al (L=100 .mu.m, W=5000 .mu.m): (a) transfer plots for ZnO
nanorod-based (circle symbols) and nanocomposite-based (square
symbols) TFTs; (b) output plot for a representative ZnO
nanocomposite-based TFT; and (c) output plot for a representative
ZnO nanorod-based TFT.
FIG. 13 compares the conductivities of nanoparticle-based versus
nanocomposite-based ITO thin films (a) without and (b) with H.sub.2
treatment measured under air as a function of the processing
temperature.
FIG. 14 illustrates four different configurations of thin film
transistors.
FIG. 15 illustrates two different configurations of
bulk-heterojunction thin film photovoltaic devices (also known as
solar cells).
DETAILED DESCRIPTION
Throughout the application, where compositions are described as
having, including, or comprising specific components, or where
processes are described as having, including, or comprising
specific process steps, it is contemplated that compositions of the
present teachings also consist essentially of, or consist of, the
recited components, and that the processes of the present teachings
also consist essentially of, or consist of, the recited process
steps.
In the application, where an element or component is said to be
included in and/or selected from a list of recited elements or
components, it should be understood that the element or component
can be any one of the recited elements or components, or can be
selected from a group consisting of two or more of the recited
elements or components. Further, it should be understood that
elements and/or features of a composition, an apparatus, or a
method described herein can be combined in a variety of ways
without departing from the spirit and scope of the present
teachings, whether explicit or implicit herein.
The use of the terms "include," "includes", "including," "have,"
"has," or "having" should be generally understood as open-ended and
non-limiting unless specifically stated otherwise.
The use of the singular herein includes the plural (and vice versa)
unless specifically stated otherwise. In addition, where the use of
the term "about" is before a quantitative value, the present
teachings also include the specific quantitative value itself,
unless specifically stated otherwise. As used herein, the term
"about" refers to a .+-.10% variation from the nominal value unless
otherwise indicated or inferred.
It should be understood that the order of steps or order for
performing certain actions is immaterial so long as the present
teachings remain operable. Moreover, two or more steps or actions
may be conducted simultaneously.
The present teachings, in part, provide a method of preparing a
metal oxide thin film which, when used as an electrically
transporting or insulating component in a semiconductor device, can
provide various advantages. For example, depending on the
composition of the metal oxide, the metal oxide thin film can be
used as one or more of the semiconductor components, one or more of
the dielectric components, or one or more of the conductor
components in a semiconductor device. In certain embodiments, the
present method can be used to prepare multiple metal oxide thin
films of different compositions, where the metal oxide thin films
of different compositions can be used as different components in a
semiconductor device. In particular embodiments, the present method
can be used to fabricate a transistor which includes a
semiconductor component, a dielectric component, and conductor
components, where at least one of these components, at least two of
these components, or each of these components can be composed of a
metal oxide thin film prepared according to the present methods. In
other embodiments, the present method can be used to fabricate a
photovoltaic cell which includes a semiconductor component, at
least one of the conductor components, and/or at least one of the
interlayer components that is composed of a metal oxide thin film
prepared according to the present methods.
Accordingly, in one aspect, the present teachings can be directed
to a method of preparing a metal oxide thin film for use as an
electrically transporting component (e.g., a semiconductor
component or a conductor component) or an electrically insulating
component (e.g., a dielectric component) in a semiconductor device.
The method generally includes depositing a thin film from a
precursor composition as described herein, and annealing the thin
film at a temperature less than or about 350.degree. C. to form a
metal oxide thin film. While various precursors have been used in
existing solution-phase methods for processing metal oxide thin
films, conventional precursors (e.g., a sol-gel system including a
metal source, a base catalyst, a stabilizer, and a solvent as
described above) typically require high temperatures
(.gtoreq.400.degree. C.) to complete the condensation of the
precursor sol, the removal of the organic stabilizer within the
thin film, and finally, full densification of the metal oxide thin
films. Such high-temperature requirements by any of these steps can
risk complications such as film cracking induced by thermal
expansion coefficient mismatch. Further, such high processing
temperature requirements make these methods incompatible with most
conventional flexible plastic substrates. Even in limited cases
where the processing temperatures can be lowered, the reported
mobilities achieved by the resulting metal oxide thin film
semiconductor are limited (.about.1 cm.sup.2/Vs).
By comparison, the precursor compositions according to the present
teachings include a redox pair of precursors (a fuel and an
oxidizing agent) that are chosen and provided under conditions to
induce a combustion reaction. Specifically, the precursors are
selected and provided at amounts such that the fuel and the
oxidizing agent react in a series of reactions, over the course of
which heat is generated, and the fuel is oxidized by the oxidizing
agent and mostly converted into gases such as CO.sub.2, H.sub.2O,
and N.sub.2. The self-generated heat from the precursors' reaction
provides a localized energy supply, thereby eliminating the need
for high, externally applied processing temperatures. Thus, the
temperature requirement of the annealing step can be lowered
generally to less than about 400.degree. C., preferably about
350.degree. C. or less, about 300.degree. C. or less, about
250.degree. C. or less, about 200.degree. C. or less, or as low as
about 150.degree. C. Despite the use of lower annealing
temperatures, the choice of the precursors and the combustion
reactions they enable result in metal oxide thin films with
desirable microstructural and molecular features, which confer
satisfactory electronic properties for the films to be useful as
electrically transporting or insulating components in semiconductor
devices. For example, when incorporated into a thin film
transistor, a metal oxide semiconductor prepared by the present
method can exhibit enhanced field effect characteristics such as
increased field effect mobilities (e.g., >1 cm.sup.2/Vs).
Specifically, as demonstrated by the examples hereinbelow,
mobilities as high as .about.40 cm.sup.2/Vs were achieved by
transistor devices implementing the present metal oxide thin films
as the semiconductor, where the annealing temperature was only
about 250.degree. C. Even with an annealing temperature as low as
about 200.degree. C., mobilities over 10 cm.sup.2/Vs were achieved.
Similarly, a conductivity greater than about 100 S/cm was obtained
with transparent conducting metal oxide thin films prepared by the
present methods at an annealing temperature of only about
250.degree. C. Thus, the low processing temperatures according to
the present teachings can enable fabrication of high-performance
devices on conventional flexible plastic substrates that have a low
heat-resistance.
Accordingly, in one aspect, the present teachings relate to
precursor compositions that can be used to prepare various metal
oxide thin films via solution-phase processes including various
thin film metal oxide semiconductors, thin film metal oxide
conductors, and thin film metal oxide dielectrics. Exemplary
semiconducting metal oxides include indium oxide (In.sub.2O.sub.3),
indium zinc oxide (IZO), zinc tin oxide (ZTO), indium gallium oxide
(IGO), indium-gallium-zinc oxide (IGZO), tin oxide (SnO.sub.2),
nickel oxide (NiO), copper oxide (Cu.sub.2O), and zinc oxide (ZnO).
These semiconducting films can have dopants (such as fluorine,
sulfur, lithium, rhodium, silver, cadmium, scandium, sodium,
calcium, magnesium, barium, and lanthanum) to improve electron (for
n-type) or hole (for p-type) mobility and/or conductivity.
Exemplary insulating metal oxides include alumina
(Al.sub.2O.sub.3), cerium oxide (CeO.sub.x), yttrium oxide
(Y.sub.2O.sub.3), titanium oxide (TiO.sub.2), zirconium oxide
(ZrO.sub.2), hafnium oxide (HfO.sub.2), tantalum oxide
(Ta.sub.2O.sub.5), and barium and strontium titanium oxide
((Ba,Sr)TiO.sub.3). Exemplary conducting metal oxides include
transparent conducting oxides such as indium tin oxide (ITO, or
tin-doped indium oxide Sn--In--O where the Sn content is about 10%
or less), indium-doped zinc oxide (IZO), zinc indium tin oxide
(ZITO), gallium-doped zinc oxide (GZO), gallium-doped indium oxide
(GIO), fluorine-doped tin oxide (FTO), gallium indium tin oxide
(GITO), and aluminum-doped zinc oxide (AZO).
A precursor composition according to the present teachings
generally includes a fuel and one or more oxidizing agents in a
solvent or solvent mixture, wherein the fuel and/or at least one of
the oxidizing agent(s) comprise a metal reagent, and wherein the
fuel and the one or more oxidizing agents are provided under
conditions that would favor combustion reactions. Generally, the
fuel and the one or more oxidizing agents are present in
substantially stoichiometric amounts to allow formation of the
desired metal oxide and complete combustion of the remaining
reagents (that is, the stoichiometric ratio should be calculated
based on the ideal oxidation of all components). Despite the fact
that some variations from the stoichiometric amounts are
acceptable, when the precursor composition includes either too much
oxidant (e.g., more than ten times exceeding the stoichiometric
amount) or too much fuel (e.g., more than ten times exceeding the
stoichiometric amount), combustion reactions can be disfavored and
such precursor compositions may not allow metal oxide thin films to
be formed under the favorable thermodynamics according to
combustion chemistry, which may lead to a high level of impurities,
poor film morphology, and/or poor electrical connections within the
metal-O-metal lattice. In addition, while conventional metal oxide
precursor compositions typically include at least one metal
alkoxide, the present precursor composition does not include any
alkoxide as a metal source.
Different embodiments of the redox pair of precursors according to
the present teachings are possible. In certain embodiments, the
present precursor composition can include one or more metal
reagents, an organic fuel, and optionally an inorganic reagent,
wherein the organic fuel forms a redox pair with at least one of
the metal reagents or the inorganic reagent; that is, at least one
of the metal reagents or the inorganic reagent comprises an
oxidizing anion which can react with the organic fuel (an organic
compound) in a combustion reaction to produce CO.sub.2, H.sub.2O,
and optionally N.sub.2 and/or other gases depending on the
composition of the fuel. In other embodiments, the present
precursor composition can include a fuel and an oxidizing agent,
wherein the fuel can be in the form of a first metal reagent (i.e.,
the fuel can take the form of an anion) and the oxidizing agent can
be in the form of a second metal reagent (i.e., the second metal
reagent can include an oxidizing anion). In yet other embodiments,
the fuel can be in the form of a first metal reagent (i.e., the
fuel can take the form of an anion), and the oxidizing agent can be
an acid or an inorganic reagent comprising an oxidizing anion. In
various embodiments, the present precursor composition can include
a base, typically, NH.sub.3. In various embodiments, the base can
be introduced into the precursor composition after the fuel and the
oxidizing reagent have dissolved completely in the solvent or
solvent mixture.
Examples of oxidizing anions include, but are not limited to,
nitrates, nitrites, perchlorates, chlorates, hypochlorites, azides,
N-oxides (R.sup.3N.sup.+--O.sup.-), peroxides, superoxides,
high-valent oxides, persulfates, dinitramides, nitrocyanamides,
nitroarylcarboxylates, tetrazolates, and hydrates thereof. As
described above, in some embodiments, the oxidizing agent can be in
the form of an acid, in which case, the acid can be a corresponding
acid of one of the oxidizing anions described herein (e.g., nitric
acid). For example, the oxidizing agent can be in the form of an
acid in embodiments where the fuel is a metal reagent including a
fuel anion.
The fuel in the present precursor compositions generally can be
described as a compound or anion capable of being oxidized by the
oxidizing agent and releasing energy (i.e., heat) by the process of
oxidation. This fuel component can be decomposed into one or more
intermediates such as COO.sup.-, CO, CH.sub.4, CH.sub.3O.sup.-,
NH.sub.2NHOH, NH.sub.3, N.sub.2H.sub.3.sup.-, N.sub.2H.sub.4, and
N.sub.2H.sub.5.sup.+, before conversion into CO.sub.2, H.sub.2O,
and optionally, N.sub.2. When the fuel is an organic compound, the
organic fuel can be composed of carbon, oxygen, and hydrogen, and
in some embodiments, also nitrogen. Other elements can be present
in the fuel such as fluorine, sulfur, and phosphorus. Typically,
the organic fuel is a relatively low molecular weight compound. For
example, the organic fuel can have a molar mass of about 200 g/mol
or less. Examples of organic fuel that can be used as one of the
precursors according to the present methods include, without
limitation, acetylacetone (CH.sub.3COCH.sub.2COCH.sub.3),
fluorinated derivatives of acetylacetone (e.g.,
CF.sub.3COCH.sub.2COCF.sub.3 or CH.sub.3COCHFCOCH.sub.3), imine
derivatives of acetylacetone (e.g.,
CH.sub.3COCH.sub.2C(.dbd.NR)CF.sub.3 or
CH.sub.3C(.dbd.NR)CHFC(.dbd.NR)CH.sub.3), phosphine derivatives of
acetylacetone (e.g., Ph.sub.2POCH.sub.2COCH.sub.3), urea
(CO(NH.sub.2).sub.2), thiourea (CS(NH.sub.2).sub.2), glycine
(C.sub.2H.sub.5NO.sub.2), alanine (C.sub.3H.sub.7NO.sub.2),
N-methylurea (CH.sub.3NHCONH.sub.2), citric acid
(HOC(COOH)(CH.sub.2COOH).sub.2), stearic acid
(CH.sub.3(CH.sub.2).sub.16COOH), ascorbic acid, ammonium
bicarbonate (NH.sub.4HCO.sub.3), nitromethane, ammonium carbonate
((NH.sub.4).sub.2CO.sub.3), hydrazine (N.sub.2H.sub.4),
carbohydrazide (CO(N.sub.2H.sub.3).sub.2), oxalyl dihydrazide,
malonic acid dihydrazide, tetra formal tris azine (TFTA),
hexamethylenetetramine (C.sub.6H.sub.12N.sub.4), malonic anhydride
(OCH(CH.sub.2)CHO), as well as diamines, diols, or dioic acids
having an internal alkyl chain of 6 carbon atoms or less. In
embodiments where the fuel component also acts as the metal source,
the corresponding ester of the carboxylic acids or anhydrides
described herein can be used instead. To illustrate, examples of
fuel anions can include, without limitation, acetylacetonates
(including fluorinated, imine or phosphine derivatives thereof),
oxalates, citrates, ascorbates, stearates, and so forth. Various
metal acetylacetonates are commercially available including
aluminum (III) acetylacetonate, zinc (II) acetylacetonate, and
zirconium (IV) acetylacetonate. Indium (III) acetylacetonate,
gallium (III) acetylacetonate, and tin(II) acetylacetonate are
known in the literature, as well as various metal oxalates, metal
citrates, metal ascorbates, and metal stearates.
Depending on the composition of the desired metal oxides, one or
more metal reagents can be present in the precursor composition.
Each metal reagent can include a metal selected from a transition
metal (any Group 3 to Group 11 metal), a Group 12 metal, a Group 13
metal, a Group 14 metal, a Group 15 metal, and a lanthanide. In
certain embodiments, the present precursor composition can include
a metal reagent having a metal selected from a Group 13 metal, a
Group 14 metal, a Group 15 metal, and a lanthanide. In particular
embodiments, the present precursor composition can include at least
a Group 13 metal reagent, for example, an indium (In) reagent
and/or a gallium reagent (Ga) for preparing an electrically
transporting metal oxide such as In.sub.2O.sub.3, IZO, IGO, IGZO,
or ITO. In particular embodiments, the present precursor
composition can include a Group 13 metal (such as aluminum (Al))
and/or a lanthanide (such as lanthanum (La) or cerium (Ce)) for
preparing an insulating metal oxide such as Al.sub.2O.sub.3,
CeO.sub.x, La.sub.2O.sub.3 or LaAlO.sub.3.
To illustrate, the precursor composition according to the present
teachings can be used to prepare an indium-containing metal oxide,
for example, In.sub.2O.sub.3, IZO, IGO, IGZO, or ITO. Accordingly,
in some embodiments, a precursor composition that can be used to
prepare the indium-containing metal oxide thin film according to
the present teachings can include an organic fuel selected from
acetylacetone (including fluorinated, imine, or phosphine
derivatives thereof), urea, N-methylurea, citric acid, stearic
acid, ascorbic acid, hydrazine, carbohydrazide, oxalyl dihydrazide,
malonic acid dihydrazide, and malonic anhydride; and an indium salt
comprising an oxidizing anion selected from a nitrate, a nitrite, a
perchlorate, a chlorate, a hypochlorite, an azide, an N-oxide, a
peroxide, a superoxide, a high-valent oxide, a persulfate, a
dinitramide, a nitrocyanamide, a nitroarylcarboxylate, a
tetrazolate, and hydrates thereof. For example, the precursor
composition can include In(NO.sub.3).sub.3 or a hydrate thereof,
and an organic fuel such as acetylacetonate,
CF.sub.3COCH.sub.2COCF.sub.3, CH.sub.3COCHFCOCH.sub.3,
CH.sub.3COCH.sub.2C(.dbd.NR)CF.sub.3,
CH.sub.3C(.dbd.NR)CHFC(.dbd.NR)CH.sub.3,
Ph.sub.2POCH.sub.2COCH.sub.3, or urea. In other embodiments, an
indium-containing metal oxide thin film can be prepared according
to the present teachings using a precursor composition that can
include an indium salt including a fuel anion selected from an
acetylacetonate, an oxalate, a citrate, an ascorbate, and a
stearate; and an oxidizing agent that is either an acid or an
inorganic reagent including an oxidizing anion selected from a
nitrate, a nitrite, a perchlorate, a chlorate, a hypochlorite, an
azide, an N-oxide, a peroxide, a superoxide, a high-valent oxide, a
persulfate, a dinitramide, a nitrocyanamide, a
nitroarylcarboxylate, a tetrazolate, and hydrates thereof. For
example, the precursor composition can include indium
acetylacetonate as the fuel and either nitric acid (HNO.sub.3) or
NH.sub.4NO.sub.3 as the oxidizing agent. In yet other embodiments,
an indium-containing metal oxide thin film can be prepared
according to the present teachings using a precursor composition
that can include a first indium salt including an oxidizing anion
selected from a nitrate, a nitrite, a perchlorate, a chlorate, a
hypochlorite, an azide, an N-oxide, a peroxide, a superoxide, a
high-valent oxide, a persulfate, a dinitramide, a nitrocyanamide, a
nitroarylcarboxylate, a tetrazolate, and hydrates thereof; and a
second indium salt including a fuel anion selected from an
acetylacetonate (including fluorinated, imine, or phosphine
derivatives thereof), an oxalate, a citrate, an ascorbate, and a
stearate. In any of these embodiments, the precursor composition
can include NH.sub.3.
When mixed oxides (e.g., ternary or quaterny oxides) are desired,
the additional metal reagent(s) can comprise any anion that would
confer satisfactory solubility to the metal reagent(s) in the
solvent or solvent mixture of the precursor composition.
Accordingly, the additional metal reagent(s) independently can
comprise an oxidizing anion, a fuel anion, or a non-oxidizing
anion. Examples of non-oxidizing anions include, but are not
limited to, halides (e.g., chlorides, bromides, iodides),
carbonates, acetates, formates, propionates, sulfites, sulfates,
hydroxides, alkoxides, trifluoroacetates,
trifluoromethanesulfonates, tosylates, mesylates, and hydrates
thereof. In embodiments where a desired metal is not chemically
stable as an oxidizing salt and/or it is not readily available as a
salt comprising a fuel anion as described herein, an inorganic
reagent comprising an oxidizing anion or a fuel anion can be used.
For example, an inorganic reagent that can be used as an oxidizing
agent can be selected from ammonium nitrate, ammonium dinitramide,
ammonium nitrocyanamide, and ammonium perchlorate. Examples of
inorganic reagents that can be used as a fuel can include, without
limitation, ammonium acetylacetonate, ammonium oxalate, ammonium
ascorbate, ammonium citrate, and ammonium stearate.
In certain embodiments, the present precursor composition can
include a first metal salt and a second metal salt, wherein the
first metal salt comprises a fuel and the second metal salt
comprises an oxidizing anion. For example, the present precursor
composition can include a redox pair including a metal nitrate and
a metal acetylacetonate. Various metal acetylacetonates are
commercially available including aluminum (III) acetylacetonate,
zinc (II) acetylacetonate, and zirconium (IV) acetylacetonate.
Indium (III) acetylacetonate, gallium (III) acetylacetonate, and
tin(II) acetylacetonate are known in the literature. Other examples
of metal salts that can function as a fuel include, but are not
limited to, metal oxalates, metal citrates, metal ascorbates, metal
stearates, and so forth.
The concentration of metal reagents in the precursor composition
can be between about 0.01 M and about 5.0 M. For example, the metal
reagent can have a concentration between about 0.02 M and about 2.0
M, between about 0.05 M and about 1.0 M, between about 0.05 M and
about 0.5 M, or between about 0.05 M and about 0.25 M. In
embodiments in which the precursor composition includes two or more
metal reagents, the relative ratio of the metal reagents can vary,
but typically ranges from 1 to 10.
The solvent or solvent mixture can include water and/or one or more
organic solvents. For example, the solvent can be selected from
water, an alcohol, an aminoalcohol, a carboxylic acid, a glycol, a
hydroxyester, an aminoester, and a mixture thereof. In some
embodiments, the solvent can be selected from water, methanol,
ethanol, propanol, butanol, pentanol, hexyl alcohol, heptyl
alcohol, ethyleneglycol, methoxyethanol, ethoxyethanol,
methoxypropanol, ethoxypropanol, methoxybutanol, dimethoxyglycol,
N,N-dimethylformamide, and mixtures thereof. In particular
embodiments, the solvent can be an alkoxyalcohol such as
methoxyethanol, ethoxyethanol, methoxypropanol, ethoxypropanol, or
methoxybutanol.
In some embodiments, the precursor composition can further include
one or more additives selected from detergents, dispersants,
binding agents, compatibilizing agents, curing agents, initiators,
humectants, antifoaming agents, wetting agents, pH modifiers,
biocides, and bacteriostats. For example, surfactants, chelates
(e.g., ethylenediaminetetraacetic acid (EDTA)), and/or other
polymers (e.g., polystyrene, polyethylene,
poly-alpha-methylstyrene, polyisobutene, polypropylene,
polymethylmethacrylate and the like) can be included as a
dispersant, a binding agent, a compatibilizing agent, and/or an
antifoaming agent.
As demonstrated by the examples herein, metal oxide synthesis using
combustion chemistry between a fuel and an oxidizing agent offers
many advantages for film solution processing. First, the
availability of high local temperatures without a furnace enables
low-cost large-scale thin-film syntheses, and the high
self-generated energies can convert the precursors into the
corresponding oxides at low process temperatures. In contrast,
oxide formation via conventional precursors based on sol-gel
chemistry conversion is endothermic, and requires significant
external energy input to form metal-O-metal lattices, whereas
combustion synthesis is exothermic and does not require external
energy input once ignited. Furthermore, conventional precursors
typically require high temperatures for decomposing the organic
stabilizer to achieve phase-pure products, while in combustion
reactions with balanced redox chemistry, the atomically local
oxidizer supply can remove organic impurities efficiently without
coke formation.
In various embodiments, the precursor composition can be aged for
at least about 3 hours, for at least about 6 hours, for at least
about 9 hours, for at least about 12 hours, or for at least about
18 hours, before the film deposition step. The depositing step can
be carried out by various solution-phase methods. For example, the
depositing step can be carried out by printing, including inkjet
printing and various contact printing techniques (e.g.,
screen-printing, gravure printing, offset printing, pad printing,
lithographic printing, flexographic printing, and microcontact
printing). In other embodiments, the depositing step can be carried
out by spin-coating, slot-coating, drop-casting, zone casting, dip
coating, blade coating, spray-coating, rod coating, or
stamping.
The typical atmosphere to carry out the deposition step and/or the
annealing step is ambient air; however, other conditions are
possible including air with controlled humidity level,
oxygen-enriched atmospheres, nitrogen-enriched atmospheres, pure
oxygen, pure nitrogen, or pure hydrogen. In certain embodiments,
the deposition step and the annealing step can be carried out in
different atmospheres; for example, both steps can be carried out
under ambient air but each of the steps can be performed under
ambient air with different relative humidity levels. In certain
embodiments, the annealing step can be performed under an
oxygen-containing atmosphere with low humidity to enhance the
combustion reaction. During the annealing step, the solvent is
eliminated, and the oxidizing metal reagent and the fuel (or the
metal salt comprising the fuel and the oxidizing agent) react to
form a metal-O-metal lattice. Also, organic impurities within the
thin film are removed. As described above, the combustion chemistry
enabled by the present precursor compositions allows the annealing
temperature to be lowered to less than or about 350.degree. C. In
various embodiments, the annealing step can be carried out at a
temperature of less than or about 325.degree. C., less than or
about 300.degree. C., less than or about 275.degree. C., less than
or about 250.degree. C., less than or about 225.degree. C., less
than or about 200.degree. C., less than or about 180.degree. C., or
as low as about 150.degree. C. More generally, the annealing step
can be performed at a temperature lower than the dehydration
temperature of the desired metal oxide. Table 1 provides the
reported dehydration temperature of various metal oxides.
TABLE-US-00001 TABLE 1 Metal Hydroxide Mg(OH).sub.2 MgO
(300.degree. C.) Al(OH).sub.3 AlOOH (300.degree. C.)
Al.sub.2O.sub.3 (500.degree. C.) Si(OH).sub.4 a-SiO.sub.2
(600.degree. C.) Zn(OH).sub.2 ZnO (155.degree. C.) Ga(OH).sub.3
Ga.sub.2O.sub.3 (420-500.degree. C.) Cd(OH).sub.2 CdO (360.degree.
C.) In(OH).sub.3 In.sub.2O.sub.3 (270-330.degree. C.) Sn(OH).sub.4
SnO.sub.2 (290.degree. C.) Pb(OH).sub.2 PbO (T.sub.dehyd <
100.degree. C.) Y(OH).sub.3 YOOH (250.degree. C.) Y.sub.2O.sub.3
(400.degree. C.) H.sub.2Ti.sub.2O.sub.4(OH).sub.2
TiO.sub.2(400.degree. C.) ZrO.sub.x(OH).sub.4-2x a-ZrO.sub.2
(~200-400.degree. C.) Ni(OH).sub.2 NiO (300.degree. C.)
Cu(OH).sub.2 CuO (T.sub.dehyd < 120.degree. C.) Ce(OH).sub.4
CeO.sub.2 (T.sub.dehyd < 200.degree. C.)
For example, the annealing step can be performed at a temperature
at least 25.degree. C., at least 50.degree. C., or at least
70.degree. C. lower than the dehydration temperature of the metal
oxide. The annealing step can be carried out by various methods
known in the art, for example, by using resistive elements (e.g.,
ovens), IR radiation (e.g., IR lamps), microwave radiation (e.g.,
microwave ovens), and/or magnetic heating, for a duration as short
as about 5 seconds and up to about an hour or longer for each
annealing cycle. As discussed below and demonstrated elsewhere
herein, the reduction of annealing temperatures can be used to
achieve metal oxides with good surface morphologies,
microstructural features, and electronic properties.
In particular, because metal oxide thin film electronic materials
(regardless of their use as semiconductors, dielectrics, and
conductors) require good surface morphologies to achieve optimal
performance, the thickness of the thin film provided by each
deposition step should be less than or about 50 nm. In most
embodiments, the thickness of the thin film provided by each
deposition step should be less than or about 40 nm, less than or
about 30 nm, less than or about 25 nm, less than or about 20 nm,
less than or about 15 nm, less than or about 10 nm, or less than or
about 5 nm. Thicker films up to about 100 nm can be achieved by
multiple deposition steps. Specifically, in between each deposition
step, the thin film is annealed to achieve full densification
before a new thin film is deposited. Thinner sublayers (e.g.,
.ltoreq.20 nm) generally can favor the formation of amorphous metal
oxides; while thicker films (obtained either by a single deposition
cycle or multiple deposition cycles) can favor the formation of
crystalline metal oxides. Thicker films (e.g., >100 nm),
especially those obtained with each sublayer having a thickness of
greater than about 50 nm, also tend to have a high porosity, which
can be useful in certain applications but undesirable as transistor
components.
The present methods can be used to prepare metal oxides of various
compositions, including binary oxides and mixed oxides such as
ternary oxides. In various embodiments, the metal oxide can include
at least one Group 13 metal, at least one Group 14 metal, and/or at
least one lanthanide. In certain embodiments, the present methods
can be used to prepare a thin film semiconductor comprising an
amorphous metal oxide, particularly, an amorphous ternary or
quaternary metal oxide. For example, the amorphous metal oxide thin
film semiconductor can be selected from .alpha.-IZO, .alpha.-ZTO,
.alpha.-IGO, and .alpha.-IGZO. In certain embodiments, the present
methods can be used to prepare a thin film dielectric comprising an
amorphous metal oxide. For example, the amorphous metal oxide can
be .alpha.-alumina or .alpha.-CeO.sub.2. In certain embodiments,
the present methods can be used to prepare a thin film conductor
comprising a ternary metal oxide selected from ITO and AZO.
Accordingly, in various embodiments, the present method typically
involves (a) depositing a thin film from a precursor composition as
described herein; (b) annealing the thin film at a temperature less
than or about 350.degree. C. to form a metal oxide thin film; and
carrying out steps (a) and (b) one or more times by depositing a
new thin film on any previously deposited and annealed thin film,
where each thin film formed by a single cycle of steps (a) and (b)
has a thickness of less than or about 50 nm.
The metal oxide thin film fabricated according to the present
teachings can be used in various types of semiconductor devices.
For example, the present metal oxide thin films can be used as
transparent conducting metal oxides in light-emitting devices;
electrodes or interfacial layers (e.g., hole-transport layer (HTL)
or electron-transport layer (ETL)) in (bulk-heterojunction
(BHJ-OPV) or dye-sensitized (DSSC)) photovoltaic devices; and
semiconductors, dielectrics, and/or conductors in thin film
transistors.
Accordingly, in one aspect, the present teachings can relate to a
method of fabricating a thin film transistor. The thin film
transistor can have different configurations, for example, a
top-gate top-contact structure, top-gate bottom-contact structure,
a bottom-gate top-contact structure, or a bottom-gate
bottom-contact structure. A thin film transistor generally includes
a substrate, electrical conductors (source, drain, and gate
conductors), a dielectric component coupled to the gate conductor,
and a semiconductor component coupled to the dielectric on one side
and in contact with the source and drain conductors on the other
side. As used herein, "coupled" can mean the simple physical
adherence of two materials without forming any chemical bonds
(e.g., by adsorption), as well as the formation of chemical bonds
(e.g., ionic or covalent bonds) between two or more components
and/or chemical moieties, atoms, or molecules thereof.
The present methods of fabricating a thin film transistor can
include coupling the thin film semiconductor to the thin film
dielectric; and coupling the thin film dielectric to the thin film
gate electrode. The thin film semiconductor can be coupled to the
thin film dielectric by contacting the thin film dielectric with a
semiconductor precursor composition, wherein the semiconductor
precursor composition can include a fuel and one or more oxidizing
agents in a solvent or solvent mixture, wherein the fuel and/or at
least one of the oxidizing agent(s) comprise a metal reagent, and
wherein the fuel and the one or more oxidizing agents are present
in substantially stoichiometric amounts to allow metal oxide
formation and complete combustion. For example, the semiconductor
precursor composition can include at least one oxidizing metal
reagent and a fuel selected from acetylacetone (including
fluorinated, imine, or phosphine derivatives thereof), urea,
N-methylurea, hydrazine, malonic anhydride, and a metal
acetylacetonate. In certain embodiments, the semiconductor
precursor composition can include two or more metal reagents,
wherein at least one of the metal reagents comprises an oxidizing
anion and at least one of the metal reagents comprises a metal
selected from a lanthanide, a Group 13 metal, and a Group 14 metal.
The semiconductor precursor composition now in contact with the
thin film dielectric then can be annealed to a temperature of less
than or about 350.degree. C. to provide a metal oxide thin film
semiconductor having a thickness of less than or about 50 nm.
Following the annealing step, a new cycle including the contacting
step and the annealing step can be repeated one or more times to
increase the thickness of the final metal oxide thin film
semiconductor.
The thin film dielectric can be composed of inorganic (e.g., oxides
such as SiO.sub.2, Al.sub.2O.sub.3, or HfO.sub.2; and nitrides such
as Si.sub.3N.sub.4), organic (e.g., polymers such as polycarbonate,
polyester, polystyrene, polyhaloethylene, polyacrylate), or hybrid
organic/inorganic materials. The thin film dielectric can be
coupled to the thin film gate electrode by various methods known in
the art, including the growth of self-assembled nanodielectric
materials such as those described in Yoon et al., PNAS, 102 (13):
4678-4682 (2005), and Ha et al., Chem. Mater., 21(7): 1173-1175
(2009); and solution-processable inorganic/organic hybrid materials
as described in Ha et al., J. Am. Chem. Soc., 132 (49): 17428-17434
(2010), the entire disclosure of each of which is incorporated by
reference herein. In various embodiments, the thin film dielectric
material in contact with a metal oxide thin film semiconductor
prepared according to the present teachings can have a high
dielectric constant. For example, the thin film dielectric material
can have a dielectric constant that ranges from about 4 to about
30. Furthermore, the dielectric material can be in the form of a
bilayer, where one layer is composed of an electrically insulating
organic layer which is in contact with the metal oxide
semiconductor layer according to the present teachings and a second
electrically insulating metal oxide layer which can be deposited by
solution processing or vapor deposition such as sputtering. In such
embodiments, the organic layer can have a dielectric constant
between about 2 and about 4, and the oxide layer can have a
dielectric constant between about 4 and about 30.
In certain embodiments, the thin film dielectric can be a metal
oxide thin film prepared according to the present methods
(including a nanomaterial-derived metal oxide thin film as
described below). As demonstrated in the examples hereinbelow, the
implementation of a low-temperature amorphous metal oxide thin film
dielectric with a metal oxide thin film semiconductor prepared by
the present methods can lead to much improved
semiconductor-dielectric interface, which can enhance the
transistor performance significantly. Accordingly, in certain
embodiments, the thin film gate electrode can be contacted with a
dielectric precursor composition, where the dielectric precursor
composition can include a fuel and one or more oxidizing agents in
a solvent or solvent mixture, wherein the fuel and/or at least one
of the oxidizing agent(s) comprise a metal reagent, and wherein the
fuel and the one or more oxidizing agents are present in
substantially stoichiometric amounts to allow metal oxide formation
and complete combustion. For example, the dielectric precursor
composition can include at least one metal reagent and an organic
fuel in a solvent or solvent mixture, wherein the metal reagent and
the organic fuel form a redox pair. In particular embodiments, the
metal reagent can comprise aluminum or cerium. The dielectric
precursor composition now in contact with the thin film gate
electrode then can be annealed to a temperature of less than or
about 350.degree. C. to provide a metal oxide thin film dielectric
having a thickness of less than or about 50 nm. Following the
annealing step, a new cycle including the contacting step and the
annealing step can be repeated one or more times to increase the
thickness of the final metal oxide thin film dielectric.
The gate electrode and the other electrical contacts (source and
drain electrodes) independently can be composed of metals (e.g.,
Au, Ag, Al, Ni, Cu), transparent conducting oxides (e.g., ITO, FTO,
IZO, ZITO, GZO, GIO, GITO), or conducting polymers (e.g.,
poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)
(PEDOT:PSS), polyaniline (PANI), or polypyrrole (PPy)). In certain
embodiments, the gate electrode (and/or source and drain
electrodes) of the thin film transistor can be a metal oxide thin
film (e.g., a transparent conducting oxide such as ITO, IZO, ZITO,
GZO, GIO, or GITO) prepared according to the present methods
(including a nanomaterial-derived metal oxide thin film as
described below). Accordingly, in certain embodiments, the method
can include coupling the thin film gate electrode to a substrate by
contacting the substrate with a conductor precursor composition,
where the conductor precursor composition can include a fuel and
one or more oxidizing agents in a solvent or solvent mixture,
wherein the fuel and/or at least one of the oxidizing agent(s)
comprise a metal salt, and wherein the fuel and the one or more
oxidizing agents are present in substantially stoichiometric
amounts to allow metal oxide formation and complete combustion. In
certain embodiments, the conductor precursor composition can
include at least two metal reagents and a fuel selected from
acetylacetone (including fluorinated, imine, or phosphine
derivatives thereof), urea, N-methylurea, hydrazine, malonic
anhydride, and a metal acetylacetonate. The conductor precursor
composition now in contact with the substrate then can be annealed
to a temperature of less than or about 350.degree. C. to provide a
metal oxide thin film conductor. Following the annealing step, a
new cycle including the contacting step and the annealing step can
be repeated one or more times to increase the thickness of the
final metal oxide thin film gate electrode. As described above, any
of the annealing steps of the present methods (whether to anneal
the semiconductor precursor composition, the dielectric precursor
composition, and/or the conductor precursor composition) can
include exposure to a radiation source.
The substrate component can be selected from doped silicon, glass,
aluminum or other metals alone or coated on a polymer or other
substrate, a doped polythiophene, as well as polyimide or other
plastics including various flexible plastics. In particular
embodiments, the substrate can be a low heat-resistant flexible
plastic substrate with which prior art conventional precursors for
processing oxide thin films are incompatible. Examples of such
flexible substrates include polyesters such as polyethylene
terephthalate, polyethylene naphthalate, polycarbonate; polyolefins
such as polypropylene, polyvinyl chloride, and polystyrene;
polyphenylene sulfides such as polyphenylene sulfide; polyamides;
aromatic polyamides; polyether ketones; polyimides; acrylic resins;
polymethylmethacrylate, and blends and/or copolymers thereof. In
particular embodiments, the substrate can be an inexpensive rigid
substrate that has relatively low heat and/or chemical resistance.
For example, the present metal oxide thin films can be coupled to
an inexpensive soda lime glass substrate, as opposed to more
expensive and higher heat and/or chemical resistant glass
substrates such as quartz and VYCOR.RTM..
Accordingly, the present teachings also encompass TFT devices that
include a substrate (including a substrate-gate material such as,
but not limited to, doped-silicon wafer, tin-doped indium oxide on
glass, tin-doped indium oxide on mylar film, and aluminum on
polyethylene terephthalate), a dielectric material as described
herein deposited on the substrate/substrate-gate, a semiconductor
material deposited on the dielectric material, and source-drain
contacts. In some embodiments, the TFT can be a transparent TFT
including one or more of the following: a transparent or
substantially transparent substrate, a transparent or substantially
transparent gate conductor, a transparent or substantially
transparent inorganic semiconductor component, a transparent or
substantially transparent dielectric component, and transparent or
substantially transparent source and drain contacts. As used
herein, "transparent" refers to having at least a 90% transmittance
in the visible region of the spectrum, and "substantially
transparent" refers to having at least 80% transmittance in the
visible region of the spectrum.
In certain embodiments, the present teachings can relate to
high-performance metal oxide TFTs fabricated, for example, with the
present low temperature-processed metal oxide thin film
semiconductor (e.g., In.sub.2O.sub.3 or IGZO) on top of a
low-temperature-processed amorphous alumina gate dielectric and ITO
gate electrode, using a flexible polymer substrate.
In another aspect, the present teachings provide
nanomaterial-derived metal oxide thin films with improved surface
morphologies and contact among the nanomaterials, which in turn
lead to improved electronic properties. Generally, the present
nanomaterial-derived metal oxide thin films include a metal oxide
nanomaterial and a binder component, wherein the binder component
includes a redox reaction product of at least one oxidizing agent
and a fuel, wherein the fuel and/or the oxidizing agent(s) comprise
the same metal as the metal oxide nanomaterial. The metal oxide
nanomaterial typically is present in an amount of at least about
50% by weight based on the overall weight of the metal oxide thin
film, whereas the binder component generally is present in an
amount of at least about 10% by weight based on the overall weight
of the metal oxide thin film.
As used herein, a "nanomaterial" generally has at least one
dimension of about 300 nm or smaller. Examples of nanomaterials
include nanoparticles (which can have irregular or regular
geometries), nanospheres, nanowires (which are characterized by a
large aspect ratio), nanoribbons (which has a flat ribbon-like
geometry and a large aspect ratio), nanorods (which typically have
smaller aspect ratios than nanowires), nanotubes, and nanosheets
(which has a flat ribbon-like geometry and a small aspect ratio).
Various metal oxide nanomaterials are commercially available or can
be prepared by one skilled in the art. Table 2 below provides
examples of metal oxide nanomaterials that can be used according to
the present teachings.
TABLE-US-00002 TABLE 2 Composition Types of Nanomaterials ZnO
nanoparticle, nanorod, nanowire GZO (Gallium doped zinc oxide)
nanoparticle Ga.sub.2O.sub.3 nanoparticle In.sub.2-xGa.sub.xO.sub.3
nanoparticle In.sub.2O.sub.3 nanoparticle, nanorod, nanowire ITO
(Tin doped indium oxide) nanoparticle, nanorod, nanowire SnO.sub.2
nanoparticle, nanorod, nanowire ATO (Antimony doped tin oxde)
nanoparticle BaTiO.sub.3 nanoparticle (Ba, Sr)TiO.sub.3
nanoparticle LiNbO.sub.3 nanoparticle Fe.sub.2O.sub.3 nanoparticle
Sb.sub.2O.sub.3 nanoparticle Bi.sub.2O.sub.3 nanoparticle CuO
nanoparticle Co.sub.3O.sub.4 nanoparticle ZnFe.sub.2O.sub.4
nanoparticle PbTiO.sub.3 nanowire
The metal oxide nanomaterials can be electrically conducting,
electrically insulating, or semiconducting as described
hereinabove. The metal oxide nanomaterials can include one or more
metals selected from a transition metal (any Group 3 to Group 11
metal), a Group 12 metal, a Group 13 metal, a Group 14 metal, a
Group 15 metal, a lanthanide, and combinations thereof.
In various embodiments, the oxidizing agent can take the form of an
oxidizing anion of a metal salt or an ammonium salt. For example,
the oxidizing anion can be a nitrate, a perchlorate, a chlorate, a
hypochlorite, an azide, an N-oxide, a peroxide, a superoxide, a
high-valent oxide, a persulfate, a dinitramide, a nitrocyanamide, a
nitroarylcarboxylate, a tetrazolate, or hydrates thereof. In some
embodiments, the oxidizing agent can be an acid such as nitric
acid. The metal component of the metal salt can include a Group 12
metal, a Group 13 metal, a Group 14 metal, a Group 15 metal, a
transition metal, or a lanthanide. In various embodiments, the
metal component typically includes the same metal(s) as the
metal(s) in the metal oxide nanomaterial. In embodiments where the
metal oxide nanomaterial is a mixed oxide, the inorganic reagent
typically includes two or more metal salts, in which case, only one
of the metal salts needs to include an oxidizing anion. The other
metal salts can include either oxidizing or non-oxidizing anion(s)
which can be selected from halides (e.g., chlorides, bromides,
iodides), carbonates, acetates, formates, propionates, sulfites,
sulfates, hydroxides, trifluoroacetates,
trifluoromethanesulfonates, tosylates, mesylates, and hydrates
thereof.
The fuel as described hereinabove can be an organic compound or the
anion of a metal reagent selected to react with the oxidizing
agent(s) in an exothermic redox reaction, and in these embodiments,
to produce the same metal oxide as the metal oxide nanomaterial.
Examples of suitable organic reagents include the various organic
fuels described above, such as acetylacetone, urea, formaldehyde,
glycine, alanine, N-methylurea, citric acid, ascorbic acid, stearic
acid, ammonium bicarbonate, nitromethane, ammonium carbonate,
hydrazine, carbohydrazide, oxalyl dihydrazide, malonic acid
dihydrazide, tetra formal tris azine, hexamethylenetetramine, and
malonic anhydride. Other examples include a metal salt that
includes an acetylacetonate, an oxalate, a citrate, an ascorbate,
or a stearate. As described above, each of these organic reagents
can be decomposed into one or more reducing or reduced moieities
such as COO.sup.-, CO, CH.sub.4, CH.sub.3O.sup.-, NH.sub.2NHOH,
NH.sub.3, N.sub.2H.sub.3.sup.-, N.sub.2H.sub.4, and
N.sub.2H.sub.5.sup.+ upon oxidation. The fuel generally is present
at around 50% to about 200% of the balanced ratio of the oxidizing
agent(s) according to propellant chemistry. The balanced chemical
reaction equation can be calculated with the oxidation number of
the reagents assuming N.sub.2, CO.sub.2, HX, H.sub.2O, MO.sub.x as
the final products. For example, the relative molar ratio of the
oxidizing agent to the fuel can be about 1 with respect to balanced
chemical equation.
In some embodiments, the present nanomaterial-derived metal oxide
thin films can be prepared by depositing a first thin film layer
from a dispersion of the metal oxide nanomaterial in a solvent or
solvent mixture (a "nanomaterial dispersion"), then depositing a
second thin film layer from a binder composition that includes a
fuel and at least one oxidizing agent in a solvent or solvent
mixture, and annealing the composite film that includes the first
thin film layer and the second thin film layer. The first thin film
layer (or the "metal oxide nanomaterial film") typically is much
thicker than the second thin film layer. For example, the first
thin film layer generally can have a thickness between about 40 nm
and about 1000 nm, whereas the second thin film layer generally has
a thickness of less than or about 100 nm. In certain embodiments,
the first thin film layer can be annealed prior to the deposition
of the second thin film layer from the binder composition. In
certain embodiments, the second thin film layer can be deposited
from two or more deposition cycles, where each deposition cycle can
provide a thin film sublayer having a thickness of less than or
about 50 nm, and in particular embodiments, less than or about 5
nm. In certain embodiments, the thin film sublayers can be annealed
between each two deposition cycles of such thin film sublayers. In
other embodiments, the thin film sublayers can be air-dried between
each two intermediate deposition cycles to remove the solvent, but
no annealing is performed until the desired thickness of the second
thin film layer is achieved. In some embodiments, two or more
cycles of (a) depositing the first thin film layer (with or without
annealing), (b) depositing the second thin film layer (via a single
depositing step or multiple depositing steps with or without
annealing in between), and (c) annealing the composite film, can be
performed.
The thickness of the first thin film layer and/or the second thin
film layer can be controlled by modulating the viscosity of the
nanomaterial dispersion and/or the binder composition, as well as
varying one or more parameters of the depositing method (such as
the number of deposition cycles, the spin or printing speed, and so
forth). For example, the viscosity of the nanomaterial dispersion
can range from about 2 cP to about 10000 cP at a temperature of
about 20.degree. C., and the nanomaterial dispersion can have a
nanomaterial loading between about 1 wt % and about 50 wt %. For
example, the nanomaterial dispersion can have a nanomaterial
loading between about 5 mg/mL and about 500 mg/mL. By comparison,
the binder composition can have a viscosity ranging from about 2 cP
to about 10000 cP at a temperature of about 20.degree. C., and a
solid loading between about 1 wt % and about 50 wt %.
In other embodiments, the present nanomaterial-derived metal oxide
thin films can be prepared from a nanomaterial-binder dispersion
which includes the metal oxide nanostructure as well as the fuel
and the one or more oxidizing agents. Based on the overall weight
of the nanomaterial-binder dispersion, the metal oxide nanomaterial
can be present in an amount of at least about 20% by weight, while
the oxidizing agent(s) can be present in an amount between about 5%
by weight and about 80% by weight, and the fuel can be present in
an amount between about 2.5% by weight and about 40% by weight.
With either embodiment, the depositing step(s) can be carried out
by various solution-phase methods including printing (such as
inkjet printing, screen-printing, gravure printing, offset
printing, pad printing, lithographic printing, flexographic
printing, and microcontact printing), spin-coating, slot-coating,
drop-casting, zone casting, dip coating, blade coating,
spray-coating, rod coating, or stamping.
Various solvents can be used in the nanomaterial dispersion and in
the binder composition. For example, the solvent can be selected
from water, an alcohol, an aminoalcohol, a carboxylic acid, a
glycol, a hydroxyester, an aminoester, and a mixture thereof. In
some embodiments, the solvent can be selected from water, methanol,
ethanol, propanol, butanol, pentanol, hexyl alcohol, heptyl
alcohol, ethyleneglycol, methoxyethanol, ethoxyethanol,
methoxypropanol, ethoxypropanol, methoxybutanol, dimethoxyglycol,
N,N-dimethylformamide, and mixtures thereof. In certain
embodiments, the binder composition also can include a base such as
aqueous ammonia. One or more additives such as detergents,
dispersants, binding agents, compatibilizing agents, curing agents,
initiators, humectants, antifoaming agents, wetting agents, pH
modifiers, biocides, and/or bacteriostats also can be added to the
nanomaterial dispersion and/or the binder composition. For example,
surfactants, chelates (e.g., ethylenediaminetetraacetic acid
(EDTA)), and/or other polymers (e.g., polystyrene, polyethylene,
poly-alpha-methylstyrene, polyisobutene, polypropylene,
polymethylmethacrylate and the like) can be included as a
dispersant, a binding agent, a compatibilizing agent, and/or an
antifoaming agent.
The annealing step can be performed under different atmospheres
including ambient air, air with controlled humidity level,
oxygen-enriched atmospheres, nitrogen-enriched atmospheres, pure
oxygen, pure nitrogen, or pure hydrogen as described hereinabove.
In various embodiments, the annealing temperature for each of the
annealing steps can be less than or about 300.degree. C., less than
or about 250.degree. C., in certain cases, less than or about
250.degree. C., and in particular cases, less than or about
150.degree. C. Because of the low-temperature processing, the
present method for preparing nanomaterial-derived metal oxide thin
films can be performed on various flexible (e.g., plastic)
substrates, which typically have a low heat-resistance. The
annealing step can be carried out by conventional heat treatment
(e.g., in an oven) and/or by irradiation with a radiation source
(e.g., infrared (IR) radiation, microwave radiation).
The present nanomaterial-derived metal oxide thin films generally
have better electronic properties when compared to metal oxide thin
films prepared from identical metal oxide nanomaterials but without
the use of the present binder composition. Similarly, the present
nanomaterial-derived metal oxide thin films also generally have
better electronic properties than those prepared with other organic
or inorganic binders known in the art. As demonstrated by the
examples hereinbelow, the use of combustion precursors according to
the present teachings as a binder leads to unexpected improvements
in conductivity (for electrically conducting metal oxides), charge
mobility (for semiconducting metal oxides), or leakage current
density (for electrically insulating metal oxides). Accordingly,
the present nanomaterial-derived metal oxide thin films can be used
to provide various thin film semiconductor devices with improved
device performance. For example, the present metal oxide thin films
can be used as transparent conducting metal oxides in
light-emitting devices; electrodes in photovoltaic devices;
semiconductors, dielectrics, and/or conductors in thin film
transistors.
Accordingly, in one aspect, the present teachings can relate to a
method of fabricating a thin film transistor, for example, a
top-gate top-contact transistor, a top-gate bottom-contact
transistor, a bottom-gate top-contact transistor, or a bottom-gate
bottom-contact transistor.
To illustrate, in some embodiments, the present
nanomaterial-derived metal oxide thin films can be incorporated
into a bottom-gate thin film transistor. For example, the
bottom-gate thin film transistor can include a nanomaterial-derived
metal oxide thin film according to the present teachings as its
semiconductor component. A dispersion of a semiconducting metal
oxide nanomaterial can be deposited on a thin film dielectric
material to provide a metal oxide nanomaterial semiconductor film
in contact with the thin film dielectric, which can be optionally
annealed (for example, at a temperature of less than or about
350.degree. C., with or without exposure to a radiation source). A
semiconductor binder composition including a redox pair of
combustion precursors according to the present teachings is then
deposited on the metal oxide nanomaterial semiconductor film. The
semiconductor binder composition now in contact with the metal
oxide nanomaterial semiconductor film then can be annealed to a
temperature of less than or about 350.degree. C. to provide a metal
oxide thin film semiconductor. As described before, the
semiconductor binder composition can be deposited via one or more
deposition cycles with optional annealing treatment in between any
two deposition cycles. The oxidizing agent(s) in the semiconductor
binder composition typically include the same metal(s) as the
semiconducting metal oxide nanomaterial. For example, the metal
oxide nanomaterial can include ZnO nanoparticles, ZnO nanorods, ITO
nanoparticles, ITO nanorods, In.sub.2O.sub.3 nanoparticles,
In.sub.2O.sub.3 nanorods, Al.sub.2O.sub.3 nanoparticles, or
mixtures thereof, and the oxidizing agent can include zinc nitrate
and/or indium nitrate. The fuel can be an organic fuel such as
acetylacetone and/or urea.
The thin film dielectric can be composed of inorganic, organic, or
hybrid organic/inorganic materials (examples of which have been
provided hereinabove); and coupled to the thin film gate electrode
by methods known in the art including self-assembly of
nanodielectric materials, vapor-phase or solution-phase deposition
of inorganic or organic materials, and solution-phase deposition of
inorganic/organic hybrid materials.
However, in certain embodiments, the thin film dielectric can be a
nanomaterial-derived metal oxide thin film prepared according to
the present methods. As demonstrated in the examples hereinbelow,
the implementation of a low-temperature amorphous metal oxide thin
film dielectric with a metal oxide thin film semiconductor prepared
by the present methods can lead to much improved
semiconductor-dielectric interface, which can enhance the
transistor performance significantly. Accordingly, in certain
embodiments, the thin film gate electrode can be contacted with a
dielectric dispersion, where the dielectric dispersion can include
an electrically insulating metal oxide nanomaterial such as
Al.sub.2O.sub.3 nanoparticles or ZrO.sub.2 nanoparticles, to
provide a metal oxide nanomaterial dielectric film. A dielectric
binder composition including a redox pair of combustion precursors
according to the present teachings then can be deposited on the
metal oxide nanomaterial dielectric film. The dielectric binder
composition now in contact with the metal oxide nanomaterial
dielectric film then can be annealed to a temperature of less than
or about 350.degree. C. to provide a metal oxide thin film
dielectric. As described before, the dielectric binder composition
can be deposited via one or more deposition cycles with optional
annealing treatment in between any two deposition cycles. The
oxidizing agent(s) in the dielectric binder composition typically
include the same metal(s) as the dielectric metal oxide
nanomaterial. For example, the inorganic reagent can include
zirconium oxynitrate. The fuel can be an organic fuel such as
acetylacetone and/or urea. In certain embodiments, the thin film
dielectric can be a metal oxide thin film prepared from a
combustion precursor composition according to the present teachings
without the use of nanomaterials.
The gate electrode and the other electrical contacts (source and
drain electrodes) independently can be composed of metals,
transparent conducting oxides, or conducting polymers (examples of
which have been provided hereinabove). In certain embodiments, the
gate electrode (and/or source and drain electrodes) of the thin
film transistor can be a metal oxide thin film (e.g., a transparent
conducting oxide such as ITO, IZO, ZITO, GZO, GIO, or GITO)
prepared according to the present methods. Accordingly, in certain
embodiments, the method can include coupling the thin film gate
electrode to a substrate by contacting the substrate with a
conductor dispersion, where the conductor dispersion can include an
electrically conducting metal oxide nanomaterial such as ITO
nanoparticles or ITO nanorods, to provide a metal oxide
nanomaterial conductor film. A conductor binder composition
including a redox pair of combustion precursors according to the
present teachings then can be deposited on the metal oxide
nanomaterial conductor film. The conductor binder composition now
in contact with the metal oxide nanomaterial conductor film then
can be annealed to a temperature of less than or about 350.degree.
C. to provide a metal oxide thin film conductor. As described
before, the conductor binder composition can be deposited via one
or more deposition cycles with optional annealing treatment in
between any two deposition cycles. The oxidizing agent(s) in the
conductor binder composition typically includes the same metal(s)
as the conductor metal oxide nanomaterial. For example, the
oxidizing agent can comprise indium nitrate, while the fuel can be
an organic fuel such as acetylacetone and/or urea. For preparing an
ITO thin film nanocomposite, SnCl.sub.2 can be used as the source
of tin, optionally with an oxidizing agent such as
NH.sub.4NO.sub.3. In certain embodiments, the thin film conductor
can be a metal oxide thin film prepared from a combustion precursor
composition according to the present teachings without the use of
nanomaterials.
Examples of various substrate materials have been provided
hereinabove. As described previously, because of the
low-temperature processing requirements, the substrate can be a low
heat-resistant flexible plastic substrate with which prior art
conventional precursors for processing oxide thin films are
incompatible. Examples of such flexible substrates include
polyesters such as polyethylene terephthalate, polyethylene
naphthalate, polycarbonate; polyolefins such as polypropylene,
polyvinyl chloride, and polystyrene; polyphenylene sulfides such as
polyphenylene sulfide; polyamides; aromatic polyamides; polyether
ketones; polyimides; acrylic resins; polymethylmethacrylate, and
blends and/or copolymers thereof. In particular embodiments, the
substrate can be an inexpensive rigid substrate that has relatively
low heat and/or chemical resistance. For example, the present metal
oxide thin films can be coupled to an inexpensive soda lime glass
substrate, as opposed to more expensive and higher heat and/or
chemical resistant glass substrates such as quartz and
VYCOR.RTM..
In some embodiments, the present nanomaterial-derived metal oxide
thin films can be incorporated into a top-gate thin film
transistor. For example, the top-gate thin film transistor can
include a nanomaterial-derived metal oxide thin film according to
the present teachings as its gate dielectric component and/or gate
electrode component (and optionally also the semiconductor
component). In these embodiments, a dispersion including an
electrically insulating metal oxide nanomaterial is deposited on
the thin film semiconductor to provide a metal oxide nanomaterial
dielectric film. A dielectric binder composition including a redox
pair of combustion precursors according to the present teachings
then can be deposited on the metal oxide nanomaterial dielectric
film. The dielectric binder composition now in contact with the
metal oxide nanomaterial dielectric film then can be annealed to a
temperature of less than or about 350.degree. C. to provide a metal
oxide thin film dielectric. As described before, the dielectric
binder composition can be deposited via one or more deposition
cycles with optional annealing treatment in between any two
deposition cycles. A nanomaterial-derived metal oxide thin film
gate electrode can be formed above the thin film dielectric in an
analogous manner. In certain embodiments, the thin film
semiconductor on which the thin film dielectric is formed also can
be a nanomaterial-derived metal oxide thin film and can be prepared
analogously on a substrate.
Accordingly, the present teachings also encompass TFT devices that
include a substrate (including a substrate-gate material such as,
but not limited to, doped-silicon wafer, tin-doped indium oxide on
glass, tin-doped indium oxide on mylar film, and aluminum on
polyethylene terephthalate), a dielectric material as described
herein deposited on the substrate/substrate-gate, a semiconductor
material deposited on the dielectric material, and source-drain
contacts. In some embodiments, the TFT can be a transparent TFT
including one or more of the following: a transparent or
substantially transparent substrate, a transparent or substantially
transparent gate conductor, a transparent or substantially
transparent inorganic semiconductor component, a transparent or
substantially transparent dielectric component, and transparent or
substantially transparent source and drain contacts. As used
herein, "transparent" refers to having at least a 90% transmittance
in the visible region of the spectrum, and "substantially
transparent" refers to having at least 80% transmittance in the
visible region of the spectrum.
FIG. 14 illustrates several different configurations of TFT
devices: (a) bottom-gate top-contact structure, (b) bottom-gate
bottom-contact structure, (c) top-gate bottom-contact structure,
and (d) top-gate top-contact structure. As shown, a TFT (e.g., a
field-effect transistor, FET, or a light-emitting transistor, LET)
can include a gate dielectric component (e.g., shown as 8, 8', 8'',
and 8'''), a semiconductor component or semiconductor layer (e.g.,
shown as 6, 6', 6'', and 6'''), a gate electrode or contact (e.g.,
shown as 10, 10', 10'', and 10'''), a substrate (e.g., shown as 12,
12', 12'', and 12'''), and source and drain electrodes or contacts
(e.g., shown as 2, 2', 2'', 2''', 4, 4', 4'', and 4'''). As shown,
in each of the configurations, the semiconductor component is in
contact with the source and drain electrodes, and the gate
dielectric component is in contact with the semiconductor component
on one side and the gate electrode on an opposite side. For OLETs,
similar architecture can be adopted where the semiconductor layer
is replaced with one or more layers that individually or in
combination perform the functions of hole transport, electron
transport, and emission.
In another aspect, the present teachings can relate to a method of
fabricating a photovoltaic device including a substrate, a thin
film metal oxide anode, a photoactive component, a thin film metal
oxide cathode, and optionally one or more thin film metal oxide
interfacial layers which can be deposited between the anode and the
photoactive component and/or between the cathode and the
photoactive component. In certain embodiments, at least one of the
thin film anode and the thin film cathode can be a metal oxide thin
film conductor (prepared from a combustion precursor composition
without nanomaterials) or a nanocomposite-based metal oxide thin
film conductor (prepared with combustion precursors either as a
coating over the nanomaterial film or as binders within the
nanomaterial film) according to the present teachings. A dispersion
including an electrically conducting metal oxide nanomaterial
(e.g., ITO nanoparticles, ITO nanorods) can be deposited on the
substrate or the photoactive component to provide a metal oxide
nanomaterial conductor film. Following an optional annealing step,
a conductor binder composition including a redox pair of combustion
precursors according to the present teachings is then deposited on
the metal oxide nanomaterial conductor film. The conductor binder
composition now in contact with the metal oxide nanomaterial
conductor film then can be annealed to a temperature of less than
or about 350.degree. C. to provide a metal oxide thin film
conductor. As described before, the conductor binder composition
can be deposited via one or more deposition cycles with optional
annealing treatment in between any two deposition cycles. The
oxidizing agent(s) in the semiconductor binder composition
typically includes the same metal(s) as the semiconducting metal
oxide nanomaterial. For example, the oxidizing agent(s) can include
indium nitrate. The fuel can be an organic fuel such as
acetylacetone and/or urea. In some embodiments, the thin film metal
oxide thin film interlayer can be a metal oxide thin film
(including a nanocomposite-based metal oxide thin film conductor)
prepared according to the present teachings. For example, NiO or
.alpha.-IZO thin films can be prepared from a redox pair of
combustion precursors comprising nickel nitrate or indium nitrate
and zinc nitrate together with a fuel such as acetylacetone.
The photoactive component disposed between the thin film anode and
the thin film cathode can be composed of a blend film which
includes a "donor" material and an "acceptor" material. For bulk
heterojunction (BHJ) organic photovoltaic devices, the acceptor
material typically is a fullerene-based compound such as C60 or C70
"bucky ball" compounds functionalized with solubilizing side
chains. Specific examples include C60 [6,6]-phenyl-C61-butyric acid
methyl ester (C.sub.60PCBM) or C.sub.70PCBM. A common donor
material used in BHJ solar cells is poly(3-hexylthiophene) (P3HT),
but other conjugated semiconducting polymers suitable as donor
materials are known in the art and can be used according to the
present teachings. Exemplary polymers can include those described
in International Publication Nos. WO 2010/135701 and WO
2010/135723.
FIG. 15 shows two different structures of bulk-heterojunction (BHJ)
solar cells. As shown in FIG. 15a, the conventional configuration
includes a transparent substrate 20 (e.g., glass), a transparent
high work-function anode 22 (e.g., ITO or FTO), a low work-function
cathode 26, and a photoactive (bulk-heterojunction) layer 24
disposed between the transparent electrode and the cathode. In some
cases, there can be an interlayer 28 between the top electrode and
the photoactive layer. For example, when the cathode is Al, an
interlayer composed of LiF often is used. Additionally, between the
transparent anode 22 and the photoactive layer 24, there can be an
interlayer functioning as either a hole-transport layer (HTL) (also
known as a hole injection layer (HIL)) and/or an electron blocking
layer (EBL) 30. FIG. 15b illustrates a BHJ solar cell having an
inverted structure. Specifically, an air-stable high-work function
metal (e.g., Ag) is used as the top (back) electrode 32. The
HTL/EBL 30, instead of being an interlayer between the transparent
anode 22 and the photoactive layer 24 as in the conventional
structure, is positioned between the photoactive layer 24 and the
top electrode 32. An additional interlayer 34 functioning as either
an electron-transport layer (ETL) (also known as an electron
injection layer (EIL) or a hole blocking layer can be present.
Metal oxide thin films of the present teachings can be used as the
electrodes (anode and/or cathode) and/or the interfacial layers
(hole and/or electron transport layer).
The present metal oxide thin films also can be used to enable other
types of thin film photovoltaic devices. For example, a
dye-sensitized solar cell can include a thin film of mesoporous
anatase (TiO.sub.2) prepared according to the present teachings.
Specifically, a precursor composition including
Ti(NO.sub.3).sub.4.4H.sub.2O and a fuel such as acetylacetone or
urea can be deposited onto an FTO-coated glass substrate to provide
an anatase film having a thickness of at least about 50 nm,
followed by annealing of the film at a temperature less than or
about 350.degree. C., thereby providing a light-converting anode.
The anatase/FTO/glass plate then can be immersed in a sensitizing
dye solution (e.g., a mixture including a photosensitive
ruthenium-polypyridine dye and a solvent) to infuse the pores
within the anatase film with the dye. A separate plate is then made
with a thin layer of electrolyte (e.g., iodide) spread over a
conductive sheet (typically Pt or Pt-coated glass) which is used as
the cathode. The two plates are then joined and sealed together to
prevent the electrolyte from leaking.
In addition to thin film transistors and thin film photovoltaic
devices, the low temperature-processed metal oxide thin films
described herein can be embodied within various organic electronic,
optical, and optoelectronic devices such as sensors, capacitors,
unipolar circuits, complementary circuits (e.g., inverter
circuits), ring oscillators, and the like.
The following examples are provided to illustrate further and to
facilitate the understanding of the present teachings and are not
in any way intended to limit the invention.
Example 1
Preparation and Thermal Analysis of Metal Oxide Precursor
Compositions
All reagents were purchased from Sigma-Aldrich and used as
received.
Example 1A
Preparation of IZO precursor composition
(In.sub.0.7Zn.sub.0.3O.sub.1.35)
Indium nitrate (In(NO.sub.3).sub.3.2.85H.sub.2O, 352.2 mg) was
dissolved in 5 mL of 2-methoxyethanol, to which 0.2 mL of
acetylacetone was added. Zinc nitrate
(Zn(NO.sub.3).sub.2.6H.sub.2O, 297.5 mg) was dissolved in 5 mL of
2-methoxyethanol, to which 0.2 mL of acetylacetone was added. After
complete dissolution of the metal nitrate, 114 .mu.L of 14.5 M
NH.sub.3(aq) was added to each solution, which was then aged for 12
hours. The indium zinc oxide (IZO) precursor composition was
obtained by mixing the two component solutions at the ratio of
In:Zn=7:3 with stirring for one hour.
Example 1B
Preparation of ITO precursor composition
(In.sub.0.9Sn.sub.0.1O.sub.1.55)
Indium nitrate (In(NO.sub.3).sub.3.2.85H.sub.2O, 352.2 mg) was
dissolved in 5 mL of 2-methoxyethanol, to which 0.2 mL of
acetylacetone was added. After complete dissolution of the metal
nitrate, 114 .mu.L of 14.5 M NH.sub.3(aq) was added, and the
solution was aged for 12 hours. Tin chloride (SnCl.sub.2, 189.6 mg)
was dissolved in 5 mL of 2-methoxyethanol, to which 0.2 mL of
acetylacetone was added. After completely dissolving the
SnCl.sub.2, 57 .mu.L of 14.5 M NH.sub.3(aq) was added, and the
solution was aged for 12 hours. The indium tin oxide (ITO)
precursor composition was obtained by mixing the two component
solutions at the ratio of In:Sn=9:1 with stirring for one hour.
Example 1C
Preparation of In.sub.2O.sub.3 precursor composition
Indium nitrate (In(NO.sub.3).sub.3.2.85H.sub.2O, 352.2 mg) was
dissolved in 5 mL of 2-methoxyethanol, to which 0.2 mL of
acetylacetone was added. After complete dissolution of the metal
nitrate, 114 .mu.L of 14.5 M NH.sub.3(aq) was added, and the
precursor composition was aged for 12 hours.
Example 1D
Preparation of ZTO precursor composition
(Zn.sub.0.3Sn.sub.0.7O.sub.1.7)
Zinc nitrate (Zn(NO.sub.3).sub.2.6H.sub.2O, 297.5 mg) and urea
(100.1 mg) were dissolved in 5 mL of 2-methoxyethanol. Tin chloride
(SnCl.sub.2, 189.6 mg), urea (60.1 mg), and ammonium nitrate
(NH.sub.4NO.sub.3, 80.1 mg) were dissolved in 5 mL of
2-methoxyethanol. The two solutions were then aged for 72 hours. To
obtain the zinc tin oxide (ZTO) precursor composition, the two
component solutions were mixed at the ratio of Zn:Sn=3:7, followed
by stirring for one hour.
Example 1E
Thermal Analysis
Thermal analyses were carried out on 10-15 mg samples prepared from
evaporated precursor compositions (Examples 1A-1D) at a heating
rate of 10.degree. C. min.sup.-1 under a 20 mL min.sup.-1 air flow.
Comparative thermal analyses were carried out on dried samples
similarly obtained from evaporated conventional precursor
compositions based on metal halides, hydroxides, and/or metal
alkoxides.
In FIG. 1, thermogravimetric analysis (TGA) and differential
thermal analysis (DTA) data are plotted for various precursor
compositions of In.sub.2O.sub.3, .alpha.-ZTO
(Zn.sub.0.3Sn.sub.0.7O.sub.1.7 for combustion precursors,
Zn.sub.1.0Sn.sub.1.0O.sub.3.0 for conventional precursors),
.alpha.-IZO (In.sub.0.7Zn.sub.0.3O.sub.1.35 for combustion
precursors, In.sub.1.0Zn.sub.1.0O.sub.2.5 for conventional
precursors), and ITO (In.sub.0.9Sn.sub.0.1O.sub.1.55). The
conventional precursor compositions were prepared as described in
Yan et al., Nature, 457: 679-686 (2009) (In.sub.2O.sub.3); Seo et
al., J. Phys. D, 42: 035106 (2009) (ZnSnO.sub.3); Choi et al.,
Electrochem. Solid-State Lett., 11: H7-H9 (2008) (InZnO.sub.2.5);
and Sakanoue et al., Nature Mater., 9: 736-740 (2010)
(In.sub.0.9Sn.sub.0.1O.sub.1.55); the disclosure of each of which
is incorporated by reference herein.
As shown, all of the combustion precursor systems exhibit
substantially lower complete conversion temperatures
(T.sub.completion<200-300.degree. C.) than the conventional
systems (T.sub.completion>500-600.degree. C.). Specifically,
unlike conventional systems which exhibit broad endotherms for
oxide lattice formation and exotherms for organic impurity removal,
the combustion systems, with the exception of ZTO, exhibit a
single, intense exotherm in the DTA, which corresponds exactly to
the abrupt mass loss in the TGA. This supports that the energy from
the exothermic reaction is sufficient to drive the reaction rapidly
to completion.
Example 2
Fabrication and Characterization of Metal Oxide Thin Films
Example 2A
Film Fabrication
The In.sub.2O.sub.3, IZO, and ZTO precursor compositions (total
metal concentration=0.05 M) were spin-coated on n++ Si wafers
(Montco Silicon Technologies, Inc.) at 3500 r.p.m. for 35 seconds,
then annealed at the desired temperature (T.sub.anneal=150.degree.
C.-400.degree. C.) for 30 minutes under air. ITO films were
fabricated by spin-coating the precursor composition (total metal
concentration=0.4 M) at 2000 r.p.m. for 35 seconds, then annealed
at the desired temperature (T.sub.anneal=200.degree. C.-500.degree.
C.) for 30 minutes under air. These processes were repeated as
necessary to achieve the desired film thickness.
Example 2B
Film Characterization
Film surface morphologies were imaged with a Veeco Dimension ICON
PT AFM System and a Hitachi S-4800-II FE-SEM. Grazing incident
angle X-ray diffraction (GIAXRD) scans were measured with a Rigaku
ATX-G Thin-Film Diffraction Workstation using Cu K.alpha. radiation
coupled to a multilayer mirror. Film thicknesses were determined by
profilometry for ITO and by X-ray reflectivity or ellipsometry for
thin dielectric and semiconductor films. Optical spectra were
acquired with a Cary 5000 ultraviolet-visible-near-infrared
spectrophotometer and were referenced to uncoated Corning 1737F
glass. XPS spectra were recorded on an Omicron ESCA Probe system
with a base pressure of 8.times.10.sup.-10 mbar (UHV), using a
monochromated Al K.alpha. X-ray source at hv=1486 eV. Quantitative
secondary ion mass spectroscopic (SIMS) analysis was performed on a
MATS quadrupole SIMS instrument using a 15 keV Ga.sup.+ ion
source.
GIAXRD data (FIG. 2, a)-c)) confirm that the combustion precursor
compositions converted to the desired crystalline oxides,
In.sub.2O.sub.3 and ITO, at far lower temperatures than the
conventional precursors. In both combustion syntheses, phase-pure
bixbyite In.sub.2O.sub.3 phases were formed after precursor
ignition at about 200.degree. C.
X-ray photoelectron spectroscopy (XPS) analysis further verified
complete conversion of the combustion precursor compositions to
oxide. The O1s scans in FIG. 3 reveal the evolution of both types
of In.sub.2O.sub.3 precursor films through the annealing process,
with the characteristic metal-oxygen-metal (M-O-M) lattice feature
at 530.1 eV increasing with increasing T.sub.anneal. These results
agree well with the above GIAXRD and thermal analysis data. Without
wishing to be bound to any particular theory, the feature at 531.6
eV could be attributed to either surface or bulk In-OH species. The
dominant In-OH feature at low annealing temperatures also confirms
incomplete formation of the oxide lattice. The additional peak at
532.3 eV can be assigned to adsorbed oxygen species (H.sub.2O,
CO.sub.2, etc.).
In addition to low conversion temperatures, the present combustion
precursor compositions also can lead to much lower impurity levels
in the final oxide films as compared to conventional precursor
compositions (FIG. 4).
To obtain optimal transistor performance, the surface morphology of
the metal oxide thin films is critical. In particular, the metal
oxides need to be densified sufficiently so that the resulting thin
films are smooth, nanstructurally well-defined, strongly adherent,
conformal, fully dense and virtually pinhole-free.
To achieve the necessary surface morphology, it has been found that
the thin film layer obtained from each deposition-annealing cycle
is preferably less than about 40 nm thick. Further, after each
deposition (e.g., spin-coating) step, the metal oxide thin film
should be annealed immediately to ensure that optimal densification
occurs. Accordingly, although the present metal oxide thin films
can have a final thickness up to about 1 micron, thicker films
(e.g., thicker than about 40 nm) typically are obtained from
multiple deposition-annealing cycles, where each
deposition-annealing cycle provides a layer of less than about 40
nm thick.
Referring to FIG. 5-a), it can be seen that when a 80 nm-thick
metal oxide film was deposited from a single deposition-annealing
cycle, the resulting film has cracks and the surface is not smooth.
Surface smoothness was improved significantly when a metal oxide
film of the same thickness was deposited from four
deposition-annealing cycles, where each deposition-annealing cycle
provides a layer of about 20 nm thick (FIG. 5-b)). When a 20 nm
metal oxide film was deposited from four deposition-annealing
cycles, where each deposition-annealing cycle provides a 5 nm
layer, the resulting film was observed to be extremely smooth and
dense, with no visible cracks or pinholes.
Example 3
Device Fabrication
The electronic properties of the present metal oxide thin films
were evaluated in thin film transistors (TFTs). Top-contact
bottom-gate TFT device structure was used.
TFT performance parameters such as saturation mobility
(.mu..sub.Sat), subthreshhold swing (S), and interfacial trap
density (D.sub.it), were evaluated with the conventional
metal-oxide-semiconductor field effect transistor (MOSFET) model
described in equations (1) and (2) below:
.mu..differential..differential..times..times.dd.times..times..apprxeq..t-
imes..times..times..function. ##EQU00001##
TFT device characterization was performed on a customized probe
station in air with a Keithley 6430 subfemtometer and a Keithley
2400 source meter, operated by a locally written Labview program
and GPIB communication.
Example 3A
Si/SiO.sub.2/metal oxide/Al top-contact TFTs
Spin-coated, combustion synthesis-derived In.sub.2O.sub.3, IZO, and
ZTO thin films were deposited on p+Si/300 nm SiO.sub.2 as
semiconductors. Thermally grown 300-nm thick aluminum source and
drain elect odes (2000 .mu.m channel width, 100 .mu.m channel
length) completed the device structure. Comparative devices were
fabricated with conventional precursor-derived oxide films.
Transistor parameters (mobility, I.sub.on/I.sub.off) are summarized
in Table 3.
TABLE-US-00003 TABLE 3 Conventional Precursor Combustion Synthesis
Based Precursor Metal Mobility Metal Mobility Oxide T.sub.a
(.degree. C.) (cm.sup.2/Vs) I.sub.on/I.sub.off Oxide T.sub.a
(.degree. C.) (cm.sup.2/Vs) I.sub.on/I.sub.off In.sub.2O.sub.3*
400.degree. C. 0.7 10.sup.6 In.sub.2O.sub.3 180.degree. C. Inactive
(.mu.~10.sup.-6).sup..dagger-dbl. In.sub.2O.sub.3 200.degree. C.
Inactive 200.degree. C. 0.81 10.sup.6 250.degree. C. Inactive
(.mu.~10.sup.-4).sup..dagger-dbl. 225.degree. C. 1.81 10.sup.6
300.degree. C. 2.30 10.sup.4 250.degree. C. 3.37 10.sup.7
400.degree. C. 5.92 10.sup.2-10.sup.4 300.degree. C. 6.5 10.sup.4
325.degree. C. 9.4 10.sup.3 ZnSnO.sub.3 200.degree. C. Inactive
Zn.sub.0.3Sn.sub.0.7O.sub.1.7 200.degree. C. Inactive
(.mu.~10.sup.-4).sup..dagger-dbl. 250.degree. C. Inactive
(.mu.~10.sup.-5).sup..dagger-dbl. 225.degree. C. 0.29 10.sup.4
300.degree. C. Inactive (.mu.~10.sup.-5).sup..dagger-dbl.
250.degree. C. 1.76 10.sup.7 350.degree. C. 0.03 10.sup.4
300.degree. C. 3.03 10.sup.6 400.degree. C. 1.67 10.sup.7
350.degree. C. 7.02 10.sup.4 400.degree. C. 7.34 10.sup.3
InZnO.sub.2.5 250.degree. C. Inactive
(.mu.~10.sup.-4).sup..dagger-dbl. In.sub.0.7Zn.sub.0.3O.sub.1.35
200.degr- ee. C. Inactive (.mu.~10.sup.-3).sup..dagger-dbl.
300.degree. C. 0.22 10.sup.5 225.degree. C. 0.32 10.sup.6
350.degree. C. 1.37 10.sup.5 250.degree. C. 0.91 10.sup.6
400.degree. C. 2.14 10.sup.5 300.degree. C. 3.20 10.sup.5
400.degree. C. 9.78 10.sup.4 *Values obtained from Kim et al.,
"High performance solution-processed indium oxide thin-film
transistors," J. Am. Chem. Soc., 130: 12580-12581 (2008) (TFT with
50 nm Au electrode). .dagger-dbl.Several devices are inactive and
some devices are active with maximum mobilities in parenthesis.
FIG. 6 plots carrier mobility, .mu., against the annealing
temperature for the In.sub.2O.sub.3, ZTO, and IZO TFT devices
derived from the two types of precursors. For the conventional
precursor route, it can be seen that adequate device performance
typically is possible only for annealing temperatures greater than
that for metal oxide lattice formation or organic impurity
oxidation (i.e., .gtoreq.300.degree. C.).
However, in the combustion systems, annealing temperatures as low
as 200.degree. C. often are sufficient for good device performance.
Without wishing to be bound by any particular theory, it is
believed that the principal driving force for oxide lattice
formation in combustion synthesis derives principally from internal
chemical energy, that is, the required applied temperature can be
described as that for reaction initiation, rather than one that
must be continuously maintained to drive the reaction.
For example, referring to FIG. 1-a) and the mobilities reported in
Table 3, it can be seen that the onset of TFT function in
In.sub.2O.sub.3 is well-matched with the thermal analysis results.
By increasing the annealing temperature to about 200.degree. C., a
mobility of .about.0.81 cm.sup.2/Vs can be achieved, which is
comparable to that of a-Si:H. Generally, a continuous mobility
increase can be observed with increasing temperature. Without
wishing to be bound by any particular theory, it is believed that
this trend is related to oxygen vacancy generation pre-filling trap
sites and/or distortional relaxation reducing trap sites.
With respect to IZO, the IZO onset temperature was observed to
exceed the conversion temperature. This can be understood by
considering the following observations. From the thermal analysis
data in FIG. 1-c) and mobility trends in FIG. 6-c), IZO conversion
appears to occur at about 200.degree. C. However, the TFT mobility
increases further with temperature, well beyond 225.degree. C.
Furthermore, for a given processing temperature, the observed
mobility is significantly lower than that of In.sub.2O.sub.3 films
(FIG. 5-a)). Without wishing to be bound to any particular theory,
it is believed that these trends can be related to insufficient
oxygen vacancy generation at low processing temperatures due to
strong oxygen binding by Zn.sup.2+.
The urea-based ZTO systems evidence slightly different trends in
the thermal analysis data. Specifically, the ZTO DTA scan reveals a
sharp, intense exotherm at 110.degree. C. and a small broad
endotherm at around 250.degree. C., with corresponding abrupt and
gradual mass losses (FIG. 1-b)). GIAXRD analysis (FIG. 2-e))
reveals that the film is amorphous up to 400.degree. C. However,
these ZTO films undergo conversion to a metal oxide semiconductor
at around 225.degree. C., judging from the TFT response shown in
FIG. 6-b). Specifically, even below complete conversion at about
250.degree. C., the TFTs exhibit a mobility of about 0.4
cm.sup.2/Vs near 225.degree. C. Without wishing to be bound by any
particular theory, it is believed that a "chemical oven" effect is
at play where an ignition between the low-temperature ignitable
urea-ammonium nitrate pair exotherm is coupled to a weakly
endothermic Zn--Sn--O reaction at higher temperatures and drives
the reaction to completion if the ignition generates sufficient
heat. Thus, upon exposing the present ZTO films even to moderate
temperatures (.about.225.degree. C., which is significantly lower
than conventional precursor systems), the internal chemical energy
combined with external thermal energy is sufficient to induce
complete oxide conversion.
Example 3B
Sn-Doped Indium Oxide Transparent Conducting Oxide
Sn-doped In.sub.2O.sub.3(In.sub.0.9Sn.sub.0.1O.sub.1.55) thin films
were prepared as follows: An ITO precursor composition according to
Example 1B having a total metal concentration of about 0.4 M was
spin-coated on 300 nm SiO.sub.2/Si at 2000 r.p.m. for 35 seconds,
then annealed at the desired temperature (T.sub.anneal=200.degree.
C.-500.degree. C.) for 30 minutes under air. This process was
repeated up to the desired thickness. For final annealing, the ITO
films were annealed for one hour under hydrogen at the same
temperature as the air annealing step for
T.sub.anneal<300.degree. C. or at 300.degree. C. for
T.sub.anneal>300.degree. C.
Conductivities of the ITO thin films on 300 nm SiO.sub.2/Si were
measured with a Keithley 2182A nanovoltmeter and 6221 current
source using the four-probe method. The results are summarized in
Table 4, along with comparison values obtained with conventional
sol-gel type In.sub.0.9Sn.sub.0.1O.sub.1.55 precursors reported in
Alam et al., "Investigation of annealing effects on sol-gel
deposited indium tin oxide thin films in different atmosphere,"
Thin Solid Films, 420: 76-82 (2002), the disclosure of which is
incorporated by reference herein.
The thermal analysis and GIAXRD data in FIG. 1-d) and FIG. 2-c)
support that crystalline ITO can be formed at temperatures as low
as 200.degree. C. from the combustion precursor systems. Because
the ITO precursor system is highly oxidizing and was used under
air, mild post-annealing treatment in a reducing environment was
required to obtain high ITO carrier concentrations. By maintaining
the combustion-derived ITO films under an H.sub.2 atmosphere either
at the same temperature as T.sub.anneal (when
T.sub.anneal<300.degree. C.) or at 300.degree. C. (when
T.sub.anneal>300.degree. C.), significant carrier densities were
achieved. FIG. 7 compares the conductivities of ITO films derived
from combustion precursors versus the conductivities of ITO films
derived from conventional precursors at different annealing
temperatures.
TABLE-US-00004 TABLE 4 Combustion Synthesis Conventional Precursor
Based Precursor T.sub.a Conductivity T.sub.a Conductivity Metal
Oxide (.degree. C.) (S/cm) Metal Oxide (.degree. C.) (S/cm)
In.sub.0.9Sn.sub.0.1O.sub.1.55 In.sub.0.9Sn.sub.0.1O.sub.1.55
200.degree- . C. 0.35 (ITO) (ITO) 225.degree. C. 15 250.degree. C.
130 300.degree. C. ~100 300.degree. C. 140 400.degree. C. ~250
400.degree. C. 440 500.degree. C. ~1200 500.degree. C. 680
For low annealing temperatures of about 200.degree. C., the film
conductivity obtained was about 0.35 S/cm. Considering the GIAXRD
evidence for complete conversion to crystalline ITO at 200.degree.
C. (FIG. 2-c)), this modest conductivity can be attributed to
insufficient free carrier concentrations at these low temperatures.
However, by increasing the annealing temperature slightly to about
250.degree. C., a conductivity of about 130 S/cm was obtained,
which is comparable to that of the conducting corrosive polymer
PEDOT:PSS. Also, it should be noted that an annealing temperature
of about 250.degree. C. or lower makes the present ITO films
thermally compatible with commercial high-temperature polymer
substrates such as polyether ether ketones, polyimides, and
polyarylates.
A continuous increase in conductivity with increasing annealing
temperature was observed for the solution-processed ITO, up to
about 680 S/cm for T.sub.anneal=500.degree. C. While these values
are insufficient for transparent data bus lines requiring
metal-like conductivity, they suffice for TFT contact electrodes.
It should be noted that solution-processed electrodes based on Au
or Ag typically show poor contact resistance for oxide
semiconductors versus Al or ITO. Because ITO is an excellent
contact material for n-type oxide semiconductors, obtaining such
contacts via low-temperature solution processing can be
advantageous.
Example 3C
n.sup.++Si/.alpha.-Alumina/in.sub.2O.sub.3/Al Top-Contact TFTs
This example demonstrates the integration of a low-temperature
solution-processed amorphous alumina dielectric with a
combustion-processed In.sub.2O.sub.3 thin film semiconductor.
Amorphous alumina dielectric films were spin-coated from a
precursor solution composed of Al(NO.sub.3).sub.3.9H.sub.2O in
2-methoxyethanol (total metal concentration=0.1 M) on n.sup.+-Si,
then annealed at 200.degree. C. or 250.degree. C. for 30 minutes
for each layer with an initial one-minute oxygen plasma treatment.
This process was repeated up to the desired thickness.
Semiconducting In.sub.2O.sub.3 thin films were deposited at the
lowest combustion temperatures as described in Example 3A. The
leakage current and capacitance of .alpha.-alumina were measured
with a Keithley 6430 subfemtometer and an HP4192A LF using
thermally evaporated 50 nm thick Au electrodes (200 .mu.m.times.200
.mu.m).
As shown in FIG. 8, .alpha.-alumina films deposited at 250.degree.
C. and 200.degree. C. exhibit far higher capacitances, 188
nF/cm.sup.2 and 173 nF/cm.sup.2, respectively (measured at 10 kHz)
compared to a 300 nm SiO.sub.2 dielectric, which has a reported
capacitance value of about 11 nF/cm.sup.2. The .alpha.-alumina
films also exhibit low leakage currents (.about.10.sup.-7
A/cm.sup.2 at 1 MV/cm) at low operating voltages (.about.2 V), with
minimal frequency sensitivity of the capacitance (.about.10%
difference from 1 kHz to 1 MHz).
In addition, a significant TFT mobility enhancement was observed
upon changing the dielectric from SiO.sub.2 to .alpha.-alumina
(FIG. 9-b), Table 5 below). For example, for
T.sub.anneal=250.degree. C., .mu..sup.sat increased from 3.4
cm.sup.2/Vs (SiO.sub.2) to 39.5 cm.sup.2/Vs (.alpha.-alumina).
Significant reduction in interfacial trap density (D.sub.it) from
2.1.times.10.sup.12 cm.sup.-2 eV.sup.-1 (SiO.sub.2) to
5.9.times.10.sup.11 cm.sup.-2eV.sup.-1 (.alpha.-alumina), as
calculated from the subthreshhold swing (S) data, indicates that
the principal origin of the increased performance is the improved
semiconductor-dielectric interface.
Example 3D
In.sub.2O.sub.3/38 nm .alpha.-Alumina/250 nm ITO/1737F Glass
Top-Contact TFTs
This example demonstrates that all-oxide solution-processed TFTs
can be fabricated with annealing temperatures at or below about
250.degree. C. Specifically, a solution-processed combustion
precursor-derived oxide semiconductor (Example 3A,
In.sub.2O.sub.3), a solution-processed combustion precursor-derived
oxide gate electrode (Example 3b, ITO) and a solution-processed
combustion precursor-derived oxide dielectric (Example 3C,
.alpha.-alumina) were combined on a glass substrate (Corning
1737F). For completion of this top-contact/bottom-gate structure,
30 nm Al source and drain electrodes of 2000 .mu.m.times.100 .mu.m
channel dimensions were defined by thermal evaporation.
Performance of the resulting TFT was found to be comparable to that
of devices fabricated on highly conducting n++Si gate electrodes
with .mu.=36.2 cm.sup.2/Vs.sup.-1 and I.sub.on/I.sub.off
.about.10.sup.4 (FIG. 9, c) and d)). Such a device also shows high
optical transparency in the visible region (T>80%).
Example 3E
Flexible Combustion-Processed Oxide TFTs
The ultimate goal of solution processing is to realize
high-throughput oxide devices on flexible substrates. To
demonstrate the efficacy of the present combustion approach for
flexible circuitry, a transparent AryLite.TM. polyester substrate
(A200HC from Ferrania Technology) was investigated as an example of
a suitable flexible substrate. A 30 nm thick Al gate (1000 .mu.m
wide, vacuum-deposited) and an .alpha.-alumina dielectric were
deposited in sequence on the AryLite.TM. substrate. The
In.sub.2O.sub.3 semiconductor was spin-coated from a combustion
precursor composition on the .alpha.-alumina as described
above.
FIG. 10 shows transfer and output plots of the resulting device.
The resulting In.sub.2O.sub.3 device on the flexible AryLite.TM.
substrate affords reasonable performance with .mu..sup.sat=6.0
cm.sup.2/Vs and I.sub.on/I.sub.off.about.10.sup.3.
Table 5 summarizes the electrical properties of combustion
precursor-derived spin-coated In.sub.2O.sub.3 semiconducting films
on various substrates using 30 nm Al source/drain electrodes.
TABLE-US-00005 TABLE 5 Mobility V.sub.TH S D.sub.it Substrate
T.sub.a (.degree. C.) (cm.sup.2/Vs) I.sub.on/I.sub.off (V) (V/dec)
(cm.sup.-2eV.sup.-1) 300 nm SiO.sub.2/p+ Si 200 0.81 10.sup.6 17.6
2.9 3.3 .times. 10.sup.12 250 3.37 10.sup.7 16.5 1.9 2.1 .times.
10.sup.12 alumina/n++ Si 200 12.6 10.sup.4 0.54 0.14 1.5 .times.
10.sup.12 250 39.5 10.sup.5 0.43 0.09 6.1 .times. 10.sup.11
alumina/ITO/1737F 250 36.2 10.sup.4 0.00 0.10 8.1 .times. 10.sup.11
alumina/Al/Arylite 200 5.97 10.sup.3 0.34 0.15 1.7 .times.
10.sup.12
Example 3F
Inkjet-Printed Combustion-Processed Oxide TFTs
This example demonstrates that combustion-processed metal oxide
thin films can be solution-processed via techniques other than
spin-coating. Specifically, inkjet-printed combustion-processed
oxide TFTs were demonstrated by printing In.sub.2O.sub.3
semiconductor channels (500 .mu.m.times.5 mm lines) on an
n++Si/.alpha.-alumina dielectric substrate with a Fujifilm Dimatix
Materials Printer DMP-2800.
Example 4
Fabrication and Characterization of IGZO TFTs
Example 4A
Preparation of IGZO (.alpha.-in.sub.6GaZn.sub.3O.sub.13.5)
Precursor Composition
All reagents were purchased from Sigma-Aldrich and used as
received. A 0.05 M IGZO precursor solution with an In:Ga:Zn metal
ratio of 6:1:3 was prepared by dissolving indium nitrate
(In(NO.sub.3).sub.3.2.85H.sub.2O, 211.13 mg), zinc nitrate
(Zn(NO.sub.3).sub.2.6H.sub.2O, 89.2 mg), and gallium nitrate
(Ga(NO.sub.3).sub.3.5.5H.sub.2O, 35.5 mg) in 20 mL of
2-methoxyethanol, then adding 204 .mu.L of acetylacetone. After
complete dissolution of the metal nitrates, 114 .mu.L of 14.5 M
NH.sub.3 (aq) was added, and the solution was aged for 12
hours.
Example 4B
Device Fabrication and Characterization
Top-contact bottom-gate IGZO TFTs based on an n.sup.+-Si/300 nm
SiO.sub.2/IGZO/Al configuration were fabricated and characterized.
IGZO precursor solutions, with a total metal concentration of 0.05
M, were spin-coated on 300 nm SiO.sub.2 wafers (WRS Materials) at
3500 r.p.m. for 35 seconds in a low humidity air environment
(RH<5%), then annealed at the desired temperature
(T.sub.anneal=250.degree. C.-350.degree. C.) for 30 minutes. This
process was repeated four times to achieve the desired film
thickness. For the final post-deposition annealing step, the IGZO
films were annealed at the temperature used to anneal each layer
for 0.5-4 hours under an atmosphere with different relative
humidities.
It was observed that by performing the post-deposition annealing
step in a high relative humidity environment, the device showed
higher mobility as well as lower hysteresis (FIG. 11).
Specifically, the mobility increased from about 1 cm.sup.2/Vs
(RH=20%) to about 3 cm.sup.2/Vs (RH=85%).
Top-gate bottom-contact OTFTs were fabricated on glass/ITO. ITO was
patterned using lithography processes affording devices having
channel length of 10 .mu.m and width of 1000 .mu.m. IGZO precursor
solutions, with a total metal concentration of 0.05 M, were
spin-coated on the substrates at about 3000-4000 r.p.m. for 35
seconds in a low humidity air environment (RH<5%), then annealed
at the desired temperature (T.sub.anneal=250.degree. C.-350.degree.
C.) for about 30 minutes. A solution of a polyolefin dissolved in
toluene was spin-coated at 1000 r.p.m. and annealed at 150.degree.
C. for 10 minutes to serve as an organic dielectric layer. These
devices were completed by thermally evaporating a 50-nm thick gold
gate electrode. The mobility of these devices was .about.4
cm.sup.2/Vs.
Top-gate bottom-contact OTFTs were fabricated on glass/ITO. ITO was
patterned using lithography processes affording devices having
channel length of 10 .mu.m and width of 1000 .mu.m. IGZO precursor
solutions, with a total metal concentration of 0.05 M, were
spin-coated on the substrates at about 3000-4000 r.p.m. for 35
seconds in a low humidity air environment (RH<5%), then annealed
at the desired temperature (T.sub.anneal=250.degree. C.-350.degree.
C.) for about 30 minutes. A bilayer composed of parylene film
(dielectric constant .about.3) in contact with ZrO.sub.x
(dielectric constant .about.25) served as the gate dielectric. The
parylene film was deposited from the vapor phase according to known
procedures. ZrO.sub.x was sputtered and the devices were then
post-annealed. The total dielectric thickness was about 200 nm.
These devices were completed by thermally evaporating a 50-nm thick
silver gate electrode. The mobility of these devices was .about.4.5
cm.sup.2/Vs.
Top-gate bottom-contact OTFTs were fabricated on glass/ITO. ITO was
patterned using lithography processes affording devices having
channel length of 10 .mu.m and width of 1000 .mu.m. IGZO precursor
solutions, with a total metal concentration of 0.05 M, were
spin-coated on the substrates at about 3000-4000 r.p.m. for 35
seconds in a low humidity air environment (RH<5%), then annealed
at the desired temperature (T.sub.anneal=250.degree. C.-350.degree.
C.) for about 30 minutes. A bilayer composed of a UV-curable
material (dielectric constant .about.3.5) film in contact with
ZrO.sub.x (dielectric constant .about.25) served as the gate
dielectric. The UV-curable material was spin-coated from
cyclopentanone and UV-cured at 300 mJ/cm.sup.2. ZrO.sub.x was
sputtered and the device were then post-annealed. The total
dielectric thickness was about 300 nm. These devices were completed
by thermally evaporating a 50-nm thick silver gate electrode. The
mobility of these devices was .about.2 cm.sup.2/Vs.
Example 5
Fabrication and Characterization of ZnO TFTs
Example 5A
Preparation of ZnO Precursor Composition
All reagents were purchased from Sigma-Aldrich and used as
received. A 0.05 M ZnO precursor solution was prepared by
dissolving zinc nitrate (Zn(NO.sub.3).sub.2.6H.sub.2O, 148.7 mg) in
10 mL of 2-methoxyethanol, then adding 100 .mu.L of acetylacetone.
After complete dissolution of the zinc nitrate, 57 .mu.L of 14.5M
NH.sub.3 (aq) was added, and the solution was aged for 12
hours.
Example 5B
Device Fabrication and Characterization
Top-contact bottom-gate ZnO TFTs based on an n.sup.+-Si/300 nm
SiO.sub.2/ZnO/Al configuration were fabricated and characterized.
The ZnO precursor solution was spin-coated on 300 nm SiO.sub.2
wafers (WRS Materials) at 3500 r.p.m. for 30 seconds, then annealed
at the desired temperature (T.sub.anneal=200.degree. C.-400.degree.
C.) for 30 minutes. This process was repeated four times to achieve
the desired film thickness.
The obtained films showed poor morphology.
Example 6
Fabrication and Characterization of Organic Solar Cells
Example 6A
Organic Solar Cells with Combustion-Derived NiO as Interfacial
Electron-Blocking/Hole-Transport Layer
In this example, bulk-heterojunction ITO/P3HT:PCBM/LiF/Al solar
cells were fabricated with combustion-derived NiO as an interfacial
electron-blocking/hole-transporting layer. Comparative devices were
made with poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate)
(PEDOT:PSS) instead of NiO.
Specifically, NiO precursor compositions were prepared by
dissolving nickel nitrate (Ni(NO.sub.3).sub.2.6H.sub.2O, 290.8 mg)
and urea (NH.sub.2CONH.sub.2, 100.1 mg) in 5 mL of
2-methoxyethanol. After complete dissolution of the metal nitrate
and urea, the solutions were aged for 12 hours.
Before device fabrication, patterned ITO-coated glass substrates
were cleaned by ultrasonic treatment in detergent, de-ionized
water, acetone, and isopropyl alcohol sequentially, and UV-ozone
treatment for 40 minutes. Then, the NiO precursor composition was
spin-coated on the ITO-coated glass substrates at 2000-4000 r.p.m.
for 35 seconds, followed by annealing at the desired temperature
(T.sub.anneal=150.degree. C.-400.degree. C.) for 30 minutes under
air. The process was repeated as necessary to achieve the desired
film thickness. For comparative devices, a PEDOT:PSS layer of about
40 nm thickness was spin-coated from an aqueous solution (Clevios P
VP AI 4083) onto ITO-coated glass substrates, followed by baking at
150.degree. C. for 15 minutes in air. A blended solution of
regioregular poly(3-hexylthiophene) (P3HT) and the fullerene
derivative [6,6]-phenyl-C.sub.61 butyric acid methyl ester (PCBM)
in a 1:1 (wt:wt) ratio was spin-coated onto either the NiO-coated
or PEDOT:PSS-coated ITO substrates. To complete the device
fabrication, a thin layer (about 0.6 nm) of lithium fluoride (LiF)
and a thin layer of aluminum (about 100 nm) were successively
deposited thermally under vacuum of .about.10.sup.-6 Torr. The
active area of the device was 0.06 cm.sup.2. The devices were then
encapsulated with a cover glass using UV curable epoxy in the glove
box.
OPV characterization was performed on a Spectra-Nova Class A Solar
Simulator with AM 1.5 G light (100 mW cm.sup.-2) from a Xe arc
lamp. The light source was calibrated with an NREL-certified Si
diode equipped with a KG3 filter to bring spectral mismatch to
unity. Current vs potential (J-V) measurements were recorded with a
Keithley 2400 digital source meter. External quantum efficiency
(EQE) was performed using an Oriel Model S3 QE-PV-SI (Newport
Instruments) equipped with an NIST-certified Si-diode and a Merlin
lock-in amplifier and optical chopper. Monochromatic light was
generated from a 300 W Xe arc lamp.
The results are reported in Table 6 below.
TABLE-US-00006 TABLE 6 V.sub.oc J.sub.sc FF PCE HTL [V]
[mA/cm.sup.2] [%] [%] 250.degree. C. NiO 10 nm .times. 2 0.504 7.78
58 2.31 10 nm .times. 1 0.476 8.53 57 2.34 5 nm .times. 2 0.483
9.07 56 2.47 5 nm .times. 1 0.533 8.53 59 2.70 400.degree. C. NiO
10 nm .times. 2 0.524 8.17 53 2.29 10 nm .times. 1 0.545 9.31 58
2.96 5 nm .times. 2 0.533 9.16 59 2.88 5 nm .times. 1 0.549 8.66 57
2.72 30 nm PEDOT 30 nm .times. 1 0.583 9.17 61 3.31
Example 6B
Organic Solar Cells with Combustion-Derived .alpha.-IZO as an
Interfacial Electron-Blocking/Hole-Transporting Layer
In this example, inverted solar cells based on an ITO/60 nm
.alpha.-InZnO.sub.2.5/P3HT:PCBM/PEDOT:PSS/Ag configuration were
fabricated.
Specifically, .alpha.-IZO precursor compositions were prepared as
follows. Indium nitrate (In(NO.sub.3).sub.2.4.12H.sub.2O, 375 mg)
was dissolved in 5 mL of 2-methoxyethanol, to which 0.2 mL of
acetylacetone was added. Zinc nitrate
(Zn(NO.sub.3).sub.2.6H.sub.2O, 297.6 mg) was dissolved in 5 mL of
2-methoxyethanol, to which 0.2 mL of acetylacetone was added. After
complete dissolution of the metal nitrate, 110 .mu.L of 14.5 M
NH.sub.3(aq) was added to each solution, which was then aged for 12
hours. The indium zinc oxide (IZO) precursor composition was
achieved by mixing the two component solutions at the ratio of
In:Zn=1:1 with stirring for one hour.
Before device fabrication, patterned ITO-coated glass substrates
were cleaned by ultrasonic treatment in detergent, de-ionized
water, acetone, and isopropyl alcohol sequentially, and UV-ozone
treatment for 40 minutes. Then, the .alpha.-IZO precursor
composition was spin-coated on the ITO-coated glass substrates at
2000 r.p.m. for 20 seconds, followed by annealing at the desired
temperature (T.sub.anneal=150.degree. C.-250.degree. C.) for about
20 minutes under air. The process was repeated as necessary to
achieve the desired film thickness. A blended solution of
regioregular poly(3-hexylthiophene) (P3HT) and the fullerene
derivative [6,6]-phenyl-C.sub.61 butyric acid methyl ester (PCBM)
in a 1:1 (wt:wt) ratio was spin-coated onto the IZO layer.
Subsequently, a PEDOT:PSS layer of about 40 nm thickness was
spin-coated from an aqueous solution (Clevios P VP AI 4083) onto
the BHJ layer, followed by baking at 150.degree. C. for 15 minutes
in air. To complete the device fabrication, a thin Ag layer was
deposited thermally under vacuum of .about.10.sup.-6 Torr. The
devices were then encapsulated with a cover glass using UV curable
epoxy in the glove box.
The photovoltaic characteristics of the encapsulated devices were
tested in air using the setup described in the previous example.
The results are reported in Table 7 below.
TABLE-US-00007 TABLE 7 T.sub.anneal V.sub.oc J.sub.sc FF PCE
(.degree. C.) [V] [mA/cm.sup.2] [%] [%] 250 0.575 10.9 53.3 3.39
225 0.562 10.9 60.0 3.68 200 0.576 10.9 62.0 3.90 175 0.574 10.9
61.6 3.84 150 0.576 10.7 55.8 3.45
Example 7
Fabrication and Characterization of Nanomaterial-Derived Metal
Oxide Thin Film Devices
To utilize the nanomaterial as pre-defined building block for
electronic application, efficient charge carrier transport between
nanomaterial is critical. This example demonstrates that the
combustion precursors can be used as efficient binders between
nanomaterials.
Example 7A
Fabrication and Characterization of ZnO Nanocomposite TFTs
ZnO nanorods (.about.60-80 nm) were synthesized following
literature procedures described in Sun et al., J. Phys. Chem. C,
111:18831 (2007), then suspended (30 mg/mL) in chloroform/methanol
(1:1). This ZnO nanorod suspension was spin-coated at 2000 r.p.m.
for 30 seconds on top of HMDS-treated 300 nm SiO.sub.2/p+Si, then
annealed at 225.degree. C. for 30 minutes under air. A ZnO
combustion precursor composition was prepared by dissolving 297.6
mg of Zn(NO.sub.3).sub.2.6H.sub.2O in 2.5 mL of 2-methoxyethanol,
to which 0.2 mL of acetylacetone was added. After complete
dissolution of the metal nitrate, 114 .mu.L of 14.5 M NH.sub.3(aq)
was added, and the solution was aged for 12 hours. This ZnO
combustion precursor composition was overcoated onto the ZnO
nanorod film by spin-coating at 2000 r.p.m. for 30 seconds,
followed by annealing at the desired temperature (200-500.degree.
C.) for 30 minutes under air. The overcoating step was performed
repeatedly to fill any voids. Aluminum source and drain electrodes
of 100 nm thickness were deposited by thermal evaporation (pressure
.about.10.sup.-6 Torr) through a shadow mask, affording channel
dimensions of 100 .mu.m (L).times.5000 .mu.m (W). Characterization
of the ZnO nanocomposite TFT devices and the control ZnO nanorod
devices (i.e., without the ZnO combustion precursor overcoating)
was performed on a customized probe station in air with a Keithley
6430 subfemtometer and a Keithley 2400 source meter, operated by a
locally written Labview program and GPIB communication. TFT
performance parameters, saturation mobility (.mu..sub.Sat) and
current on/off (I.sub.on/I.sub.off) ratio were evaluated from
transfer plots with the MOSFET model described in equation 1.
Enhanced carrier transport was confirmed with ZnO
nanocomposite-based TFTs as compared to control ZnO nanorod-based
TFTs. As shown in FIG. 12, field effect mobilities increased
significantly from 10.sup.-3-10.sup.-2 cm.sup.2/Vs (as measured
from control ZnO nanorod-based TFTs) to 0.2 cm.sup.2/Vs (as
measured from ZnO nanocomposite-based TFTs).
Example 7B
Conductivity of ITO Nanocomposite Transparent Conducting Oxide
Films
ITO nanoparticle sol (30 wt % in isopropylalcohol, <100 nm
particle size, product #700460) was purchased from Sigma-Aldrich.
This ITO nano-sol was diluted to 20 wt % in isopropylalcohol, then
spin-coated at 5000 r.p.m. for 30 seconds on 300 nm SiO.sub.2/p+Si
substrates, then annealed at the desired temperature (100.degree.
C.-400.degree. C.) for 30 minutes under air. An ITO combustion
precursor composition was prepared as follows. Indium nitrate
(In(NO.sub.3).sub.3.2.85H.sub.2O, 352.3 mg) was dissolved in 2.5 mL
of 2-methoxyethanol, to which 0.2 mL of acetylacetone was added.
After complete dissolution of the metal nitrate, 114 .mu.L of 14.5
M NH.sub.3(aq) was added, and the solution was aged for 12 hours.
Tin chloride (SnCl.sub.2, 189.6 mg) and ammonium nitrate
(NH.sub.4NO.sub.3, 80.1 mg) was dissolved in 2.5 mL of
2-methoxyethanol, to which 0.2 mL of acetylacetone was added. After
completely dissolving the inorganic salts, 57 .mu.L of 14.5 M
NH.sub.3(aq) was added, and the solution was aged for 12 hours. The
indium tin oxide (ITO) precursor composition was achieved by mixing
the two component solutions at the ratio of In:Sn=9:1 with stirring
for one hour. This ITO combustion precursor composition was
overcoated onto the ITO nanoparticle film by spin-coating at 2000
r.p.m. for 30 seconds, followed by annealing at the desired
temperature (200-500.degree. C.) for 30 minutes under air. The
overcoating step was performed repeatedly to fill any voids.
Subsequently, the ITO nanocomposite films were reduced by annealing
at either the same temperature as T.sub.anneal (if
T.sub.anneal<300.degree. C.) or at 300.degree. C. (if
T.sub.anneal.gtoreq.300.degree. C.) under a hydrogen atmosphere for
2 hours. Conductivities of the ITO nanocomposite films and the
control ITO nanoparticle films were measured with a Keithley 2400
source meter using the four-probe method.
Enhanced conductivity was confirmed with the ITO
nanocomposite-based TCO films as compared to the control ITO
nanoparticle-based TCO films. As shown in FIG. 13, the conductivity
of the nanoparticle-based ITO films increased significantly from
10.sup.-1 S/cm to about 10 S/cm by overcoating the
nanoparticle-based film with an ITO combustion precursor
composition. More remarkably, by overcoating or impregnating the
ITO nanoparticle-based films with the present combustion
precursors, a conductivity of 10 S/cm could be obtained with an
annealing temperature as low as about 150.degree. C. With
post-annealing reductive treatment, the conductivity of the ITO
nanocomposite-based TCO films further increased to 170 S/cm for
T.sub.anneal.about.200.degree. C. While the control ITO
nanoparticle-based TCO films also showed increased conductivity
with post-annealing reductive treatment, the conductivity remained
more than an order lower (<10 S/cm).
The present teachings encompass embodiments in other specific forms
without departing from the spirit or essential characteristics
thereof. The foregoing embodiments are therefore to be considered
in all respects illustrative rather than limiting on the present
teachings described herein. Scope of the present invention is thus
indicated by the appended claims rather than by the foregoing
description, and all changes that come within the meaning and range
of equivalency of the claims are intended to be embraced
therein.
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